Thanks for reaching out. First of all, please note that we strongly recommend using custom fields (of list and linked list types) over tags, since they are more flexible and you can create reports and dashboard more efficiently based on them.
As for tags, we recommend using them for temporary processes rather than permanent ones. You can create filters based on your tags in each of the modules (Requirements, Test Library, Test Sets & Runs and Issues). If you need to create a filter in one of the modules and you need specific entities to appear under that filter, you can tag theses entities, then create a filter with the tag criteria.
I hope this helps.