A post can only be in one category
But there can be tags.
I did add a tag named crossover and tagged this post which is in the imported category
https://forum.sailingresults.co.uk/t/swprize-exe-and-how-you-install-into-crossover/102
as an example and then you can search on a tag using the advanced search e.g.
but you can also search on the text such as crossover
Tags have to be created by moderators at the moment but anyone can use them
The advanced search is very powerful
Hope that helps
Jon