I looked around for a phrase that would be interesting to analyze on a blog where others are required to comment. In a short student paper posted online, I read that Japanese users not only borrow specific words, but they combine words into phrases that would otherwise have no meaning or different meaning back in the original language. The example that I found most revealing is ケーキカット which is adopted in mainstream culture to mean the ceremony of cutting the wedding cake. In English, we do not designate a specific term for this activity, so even though this is clearly a loan word from the English words "cake cut," this phrase has no significance in English. I would categorize this type of loan word as having "Slant Meaning." The original meaning of the words are kept, but a derivative of them is used in Katagana Japanese. It fits into a larger category of English turned katagana called 和製英語（わせいえいご). This category, however, is not well delineated; and includes many loan words from English that do not necessarily have a disparate meaning in Japanese.
In contrast, a loan word that doesn't fit into "Slant Meaning" is マックフライポタトthat means McDonald's French Fries. Even though it is not a direct transliteral loan, as it comes from Mac Fry Potato, the meaning is more or less direct.
A more simplified or more direct loan would be ブラシ meaning "brush" in which the transliteration, sound and meaning are simultaneously transferred.
Naturally, kanji would not be used to express these meanings as not only were these concepts first introduced to the Japanese in a non-Chinese language, but they are used phonetically in the spoken language, and so translate well into either hiragana or katagana.
So, why katagana? In Chinese there are several radicals that demonstrate a foreign or borrowed word, item or concept. These radicals signify concepts or phrases from the West or from the ancient North, and might have indicated the quality of having been imported in order to differentiate between Chinese culture from "other" cultures at a time when the state of China was struggling to unify itself as a political entity that is socially distinct from other cultural peoples. These other cultures might be Mongolians, nomadic "barbarians," Europeans, Buddhists and Persians. Perhaps, the Japanese, after adopting the Chinese language in the Tang Dynasty adopted this tradition, and has continued it through katagana instead of identifiable radicals.
To speak more of this, we can look at a few simple examples. Chinese broccoli grown in Guangdong with thick stems and long leaves is called 借兰花 but Western broccoli with its distinct flower-like top is called 西兰花 literally "Western broccoli." The same character for Western 西 is used in calling tomato, literally "Western persimmon" 西红柿. Tomato is alternatively called 番茄 or "foreign eggplant." The beloved instrument erhu played in China, Korea and Japan was conceived in the middle east, then transmitted to China then to Japan through the North, most probably from Korea. A non-native instrument, it is called 胡琴 where the first character 胡 signifies "foreign" such as in 胡人, which is most commonly read as a person who lived north of the gates of the Great Wall.
As a rule, dedicating katakana to signify foreign concepts is nowadays diluted with pop culture language trends that turn to katakana for traditionally Japanese words as well, and so the distinction is blurred. The same is true in Chinese where many young people have forgotten the origins of the words they use, and apply them to contexts where they traditionally would not have belonged.
China and Japan are not alone in their struggle to define cultural and political limits through colloquial language: so does France. To promote nationalism, which is to create the idea of and belief in a culturally unified nation with mapable borders, France has disallowed viral loan words such as "e-mail." Rather, the Academie Francaise insists that the orthodox French word "courriel" instituted in everyday dialogue. Is there a need to protect one's identity through a top-down regulation of words? In Taiwan, once a Japanese colony, Japanese loan words are integrated into Chinese (reverse kanji, perhaps). Words such as "soudesu," "skouiine" and "jyotoumade" used to replace "啊了解,” “好厉害” and “等一下” though never written, are a part of casual dialogue between friends, siblings and office coworkers.
Another possibility for the designation of katagana to foreign words or concepts is that after Japan's defeat in World War II, its government instituted a series of nationalistic measures that sought to "erase" or minimize China's historical influence on the making of Japanese language; and thus, the shaping of its history. Katagana could be used to add a layer to Japanese language that doesn't exist in traditional Kanji - phonetics, calligraphic style and short history.
Katakana textbooks are brief in their explanations of categories because katagana changes quickly, and textbooks are supposed to be used for many years. Although each text sticks to more or less one or two general ways in which katagana are used (foreign words, onomatopoeia, emphasis), there could be multiple reasons with no real rules to follow. Some of it does seem a bit on the arbitrary side as it's a conglomeration of colloquial usages. Thus, the textbook, an instrument for a non-fluent, non-native speaker, cannot accurately capture all of its uses without confusing the reader.
It's interesting to see these three levels of loan words coming into play in Katagana. Language is fluid enough that each generation has a new set of vocabulary, connotations and denotations. As the original English or Chinese meaning is distanced from users, alternative meanings are attached for a new effect. The purpose is to use existing vocabulary, and give them new meanings in new contexts for a different effect. This is one way in which language captures the evolution of a culture.
Sources:
Wikipedia
Katakana http://en.wikipedia.org/wiki/Katakana.
Persimmon http://en.wikipedia.org/wiki/Persimmon.
胡人
military.china.com/zh_cn/history2/06/.../12308456.html.
Youtube - ケーキカット
www.youtube.com/watch?v=IN0WNyfJGG4.
