If you are creating a tag file by reading in a whole C/C++ source tree, that is going to tag both the headers and the implementation files, by default. There are 3 ways you can trim this down:
1. When you first create the tag file, change the "files of type" filter to not include all the C/C++ extensions. Whittle it down to *.h;*.hpp for example.
2. Do some surgery on the existing tag file by removing specific source files. When you go to Tools > Tag Files, and select your tag file in the list on the left, the individual sources are listed on the right. You can remove files from there.
3. If you just want a handful of files, start with an empty tag file. Go to Tools > Tag Files, click New Tag File, select C/C++, then give it a name in the Open/Save dialog and click OK. But then click Cancel when the dialog comes up to browse for the source directory. That will give you a blank starter tag file. Then use the Add Files button to add individual source files to the tag database.