For starters the Arizona Fall League is usually a bit of a showcase for talents who will play in the Majors in the very near future. The level of competition is fairly high and can be a good test for prospects. However, prospects can't be Dominican or Venezuelan or any other country with a fall league of their own. So, it's basically all Americans. Teams often use AFL as a last chance to evaluate guys who may need to be added to 40-man rosters to protect them from the Rule 5 draft.
However, teams are required to supply 6-8 players, and many teams don't have 7-8 top prospects they can send, and teams can't be all Starters and Outfielders, so teams often end up sending a few Pitchers and maybe a position player or two just to fill out the roster. The Phillies supply players to the Phoenix Javelinas along with the Royals, Astros, Mariners and Padres.
This year the Phillies are sending 3 guys with looming Rule 5 eligibility in Aaron Altherr, Kelly Dugan and Cameron Rupp. Also announced as going is Kenny Giles (though he doesn't appear on the actual Javelina's roster yet), the reason for sending Giles is he missed so much of the regular season to injury that he really just needs the appearances. The Phillies then supply a good bit of the bullpen with new bullpen convert Austin Wright, who will spend the fall attempting to find home plate, while trying to miss more bats. Kyle Simon, the 6'6" righty acquired in the Thome trade will also make the trip and the Phillies contingent gets rounded out by 6'5" swingman Mike Nesseth. Nesseth has started and relieved. Thereare 4 known starters on the Javelinas roster: Brandon Maurer, Danny Hultzen, Jason Adam, Angel Baez and another swingman in Noel Arguelles. So, I'm not sure which role Nesseth will fill.
The big names on this team are Hultzen, Maurer, Delino DeShields and Cory Spangenberg. I imagine Altherr and Dugan will see plenty of time in the OF and DH roles. Rupp though is going to have a tougher time as Jake Lowery and Austin Hedges might be more highly regarded prospects. That said, the AFL teams rotate guys well and everyone gets pretty equal time to shine.