I think that these abbreviations refer to the topics.
Vocabulary in Longman Dictionary is divided into many topics, and a word may belong to more than one topic according to its meaning, so CT means that the word sweet with the first meaning belongs to the topics Cooking and Taste, CO to topics: Cooking and Odours, as for field, SE belongs to School and Education and so on, for example you will find H in all words that are related to horses, and O in occupation words. Bye the way there is a small mdx file just for topics, you can find it here: 求Longman Topic mini-Dictionary 这本词典 - #5，来自 碧落黄泉
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