the first number denotes the rim joining technique e.g
717 is a 17mm rim and the 7 denotes it is a welded rim
317 is still a 17mm rim, 3 denotes the rim is pinned or bonded not welded (e.g 117<317<717
XM819 is a 19mm rim, 8 series denotes its tubeless, XM is the range of xc/trail rims
EN/EX 521/321 are more burly all mountain/DH/FR destined rims.
I've ran 717s and 819s previously, laced on hope Pro2s, but have now gone for Stans Crests on hope pro3SP-XC3