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Hi MKnight. Welcome. Well, if you are asking about how we do categorize articles technically than i have to refer you to How do I add an article to a category?. If it was not the case than the way i proceed is the following:
I read the into of the article and find that it is about an Armoured fighting vehicle.  
I go to that article and check in which category it is contained. The answer found is Category:Armored fighting vehicles
But in this category there are other more specified sub-categories such as Category:Armored fighting vehicles by era and Category:Armored fighting vehicles by nationality
I then choose Category:Interwar armored fighting vehicles and Category:Interwar armored fighting vehicles as the more appropriate categories for the article.   - Wiki me up® "