Currently, there are two mod environment tags available: "Client" and "Server". Those then can be combined to create "Client and Server" for three tags in total.

I would like to suggest the addition of two new mod environment tags: "Optional for Client" and "Optional for Server".

Then, the two suggested tags could be combined to create three new environment tags: "Client and optional Server", "Server and optional Client" and "Optional Client and optional Server".

The newly expanded total of 6 possible tags should cover all possible combinations of environments used by mods, especially those that are not covered by the currently existing ones. The "Optional Client and optional Server" tag is intended for mods that can function on either side without being required on both (If there are any), the other 2 tags should be self explanatory.

(logically the "Optional for Client" and "Optional for Server" tags shouldn't be used by themselves as that wouldn't really make sense)

These new tags could greatly increase the end-user experience by adding some much needed extra information telling you how you can/should use each mod. The new tags would also drastically ease the manual sorting of mods to determine if you want it copied/moved over to a dedicated server or not.