More trades and specialties questions

I have read the guidebook area on them multiple times and am still not entirely understanding how things work. So i've got a couple of questions to ask about it before I go about trying to get my character into a trade/specialty. 

Is the general name of the thing the trade, or is it the specific things under the general name? (as in Mercenary is the general name, and for example Tactician being the specialty) Or does the character just go right into one of the branches of said thing? (as in the areas of major focus) 

How far in a thread do I have to go mentioning that certain thing before it is considered able to be used for that specific trade/specialty? (As in can i mention it in a post, that he is interested or training in said thing, or does every thread specifically have to be working for that specific thing. Like if I wanted to get into the Tactician area, having all threads about it with planning for dangerous situations, or if mentioning that he planned things in their free time was enough) 

When it mentions unique threads, does every single thread have to be about different things, or can it be about the same general thing and take place in different threads? Is there a minimum post count per thread to make it even count for the trade/specialty?