Creating new tags

There are a few tags I'd like to add to my files such as "subgenre" and "featured instruments" which aren't listed in mp3tag. I added them manually, which is fine, but I'm wondering if they will be visible in other programs? Is there anything I should know about tags that I've created myself? Thanks.

It is a feature of V2.3ff tags to allow user-defined tag fields.
It is hardly ever a feature of players to show user-defined tag fields (foobar does).
Some players delete user-defined fields if other fields are edited with that player.

Thanks for the response, I'm slowly learning more about metadata. It seems that these fields I'm looking for don't exist in any version of ID3, but I constantly hear composers and music libraries asking for them, so I'm a bit confused.

Which could these be?
Here is a list of fields, supported by MP3tag:

or here for the full-blown ID3 standard and its list of fields:
Perhaps you find something suitable.

I'm looking for subgenre and featured instruments, which doesn't seem to be included in mp3tag, or ID3, unless I'm not seeing it. I'm curious how other people enter these tags.

I think I got that.

so the only way around it would mis-use an existing field or add user-defined fields. User-defined fields would also be according to the standard, yet, hardly any program reads user-defined fields.
A lot of programs do not even read much more common fields like TLAN for LANGUAGE or TBPM for BPM.
There is a field "Involvedpeople" which also has the task of the participating person as part of the data. And as most instruments are played by people, that might be a good target field.
Again: hardly any player shows the data from that field.

In addition to @ohrenkino:
It heavily depends if you want to share such information with others or if you only need this information for your own use.
If you save it only for you, you can use whatever tag you like (with the explicit note that nearly no program will show it).
Otherwise you should ask the people (already using the same information) which tag they use.

Thanks for your responses. These tags are for others to use. I'm adding them to my own music that I'm submitting to music production libraries for TV and advertising. Publishers will search their catalogue based on instrumentation, mood, etc. to find the right music for their client. I don't know if they're using 2.3 or 2.4 either, so I'm not sure what to do.

I think you now have to ask the publishers in which format they want to have the data in their catalogue.
It should be clear by now that they either use a user-defined field (the name of which they could tell you) or some other field which they also could tell you.
Or they do not use tag data at all but some kind of other description, which you have to supply on top.
Whatever the requirements will be, I am sure that MP3tag will have functions to contribute.