The second part "Stretch" or "Flat" - does this refer to the tuning of the pianos sampled from?

Yes, it does. A google search of "site:yamahasynth.com stretch flat waveforms" finds an answer from Yamaha ...

Source: https://www.yamahasynth.com/ask-a-question/mono-piano

Then edit its Elements swapping the Stereo (St) Waveform for the corresponding Mono (Mn) Waveform of that same piano - if a mono version is provided. Both the Yamaha CF3 (9ft), and Yamaha S6 (6'11" ) acoustic grands come with a full compliment of Stereo and Mono piano Waveforms - both stretch tuned and non-stretched (flat) tuned.

For example, say you are using the “Full Concert Grand” - the Factory Preset is built from the “CF3 Stretch xxx St” set of Stereo Waveforms...

Your next question:

The fifth optional part is not clear - what is the "+" or "-" referring to here?

... the same message referenced above also explains + and -.

The plus (+) and minus (-) following the Waveform Name are different mappings of the same group of samples. This has an impact when combining Waveforms within an instrument Part.
+ waveforms have the samples shifted up in mapping
- waveforms have the samples shifted down in mapping.

Both are provided for the following reasons:
First, mapping the samples differently provides a slightly different timbre.
- maps will be slightly darker
+ maps will be slightly brighter.

Second, because these different Waveforms are specifically mapped so no samples overlap this allows you to layer a normally mapped wave and a shifted wave without any phasing.

Finally, with waves that have been sampled with the natural (player’s) vibrato in them, you can layer two waves and get different rates of vibrato.
- maps will have a slower vibrato than the normal map
+ maps will have faster vibrato than the normal map.

Thanks, this answers my questions. As usual, Bad Mister's answers (in the other thread - https://www.yamahasynth.com/ask-a-question/mono-piano) are very detailed and helpful.

I normally do a forum search, but I didn't get anything on this. I see you are recommending an external Google search with "site: ...". I'll try that the next time.

I always recommend using an external search index. The site is well indexed and the external engines do a much better job, generally, of finding what I'm looking for. Rarely (like when links change) I will use the built-in search only to find the new structure.