I'm no expert, but I'm going with Jerome on this one...
+ = Augmented (or sharped)
- = Diminished (or flatted)
Makes sense and is easy to remember, but tabbers have conventions also, and not everyone adheres to the guidelines.
The C+ Pix is talking about would make more sense as a C# (assuming a C Augmented chord), as a C#m would more likely (to my mind) be written as a C+-3 to reflect the flatted 3rd of the minor chord.... which would really be confusing to this old brain.
So now that we've confused everybody including me, who wants to tackle the part about "hammer-ons"? By the time I tried to put that into words, I'd be en-route to a rubber room somewhere....
Have a Happy New Year All!
Take Care;
Doug