I recognize this as a question covered a couple of times in the past.

Search engine:

site:yamahasynth.com waveform st stereo plus minus

First link returned: https://yamahasynth.com/forum/sample-waveforms-naming-convention

The previous links didn't cover "Of" - but this one does:

https://www.yamahasynth.com/forum/names-of-waveforms

”Of” is short for Offset. A Waveform with an Offset is usually one where in the original sample there was a distinct attack characteristic. A French Horn has a characteristic brass “blat” attack, the Offset Waveform would be the French Horn Sample set provided without that attack portion. Literally, it is the French Horn Waveform sans any attack. This is used in the engines ability to transition active Elements during play.

... I think this second link better covers + and - (or perhaps sometimes the meaning changes depending). This second link shows + and - are related to phase to help with cancellation if combining similar waveforms.

EDIT: updated links May 26th 2022. Also, previous suggestion to use google search engine is no longer recommended due to recent changes in indexing. The forum is not well indexed by search engines as it was before. Therefore, the recommendation is to use the search magnifying glass at the top of the forum.

For some reason, the links posted by Jason and Bill doesn't work for me.

Anyone else having this problem?

Same.

Goes to "create a new post"

[quotePost id=116966]For some reason, the links posted by Jason and Bill doesn't work for me.

Anyone else having this problem? [/quotePost]
The forum structure has changed in the meantime.

What you need to do is take those links and change "ask-a-question" with "forum".

So:

https://www.yamahasynth.com/ask-a-question/confusing-waveform-naming-convention

becomes:

https://www.yamahasynth.com/forum/confusing-waveform-naming-convention