Note that you can not actually use plain tex in latex. plain is a different format defined in plain.tex which is not input into the latex format. There are of course some macros with the same name in both plain and latex and (some) of those have the same definition, but it's not really plain tex whatever you do.
So if you want to ask how to do a floating insert and want people to answer using
\topinsert and with code that requires the command
tex rather than
latex to process it, tag your question
plain-tex but if you are asking about the use of a primitive such as
\def in latex, tag it
tex-core if you want to ask about a macro such as
\loop tag it as
latex-base or some such (
\loop being an example of a low level macro with similar but different definitions in plain and latex).
If you just want to ask a question and don't know how to tag it just tag it with anything reasonable, it doesn't matter that much anyway and the tags can always be changed once the answer and context is clearer.