Paul, most manufacturers make firearm packages that have 10rd magazines so their firearms can also be sold in states that have magazine capacity restrictions. The firearms are exactly the same but just have 10rd mags included. The firearms will also accept any standard capacity magazines just fine. Cheers and stay safe!

Paul, 10+1 is 10 rounds in the magazine and 1 in the chamber. After you rack a round in from loaded magazine, remove magazine and fill the magazine you have 10+1. Do not put a round in by hand before you put the magazine in. Load chamber from the magazine. Then remove and top off. Will break ejector if you put one in by hand then full mag then close slide.

It means the magazine holds 10 rounds, plus you can add an additional round in the chamber before inserting the magazine, that is the + 1, so it's 10 in the magazine, plus the 1 you inserted in to the chamber manually.