what does "AP VLAN ID" actually do in Access point groups?

Question

Hello, 
 We've on-boarded a client with a Sophos XG firewall and four APX530 access points. They are currently set up under an Access Point Group with VLAN tagging enabled and the AP VLAN ID set to 2. 
 
 I'm trying to figure out why VLAN 2, and what this actually does.

LuCar Toni · Accepted Answer

So basically you can give the AP a VLAN ID, which the AP will use for all "management traffic". 
 That means, the AP will communicate via VLAN ID X with the XG. 
 If you activate VLAN Tagging, the AP can only talk with VLAN IDs. 
 So the Bridge to AP LAN Mode will actually bridge everything into VLAN ID 2. 
 Only Bridge to AP VLAN will use another VLAN ID for certain SSIDs. 
 XG does not support VLAN ID 1 (which is most likely untagged).