Actually, based on their website and personal experience, it's in between the medium action (700 Short) and the long action (700 Long). That accounts for the XM nomenclature. (extended medium) These actions will accommodate cartridges up 3.2" OAL.
"Actions available in Short, Medium, XM, Long, and XL lengths to accommodate all popular factory chamberings and extended length handloads."
From the Defiance price guide:
MEDIUM ACTION (Model 700™ SA Length) – $1090 MSRP or $1190 (with scope rail) MSRP
416R SS, 1.350 OD, .700 Bolt, 4340 CM, .750 Bolt Add $100
XM ACTION – $1090 MSRP or $1190 (with scope rail) MSRP
416R SS, 1.350 OD, .700 Bolt, 4340 CM
LONG ACTION (Model 700™ LA Length) – $1140 MSRP or $1240 (with scope rail) MSRP
416R SS, 1.350 OD, .700 Bolt, 4340 CM, .750 Bolt Add $100
XL ACTION – $1240 MSRP or $1340 (with scope rail) MSRP
.750 Bolt Only, 416R, SS, 1.350 OD, 4340 CM
Regards.