Docs: Add explanation for rotation layer

docs/sources/calendar-schedules/web-schedule/create-schedule/index.md

@@ -58,6 +58,10 @@ User groups that contain
 multiple users results in all users in the group being included in corresponding shifts.
 1. When you’re satisfied with the rotation preview, click **Create**.
 
+Oncall supports multiple **layers** for rotations which allow for overlapping shifts.
+A layer defines the shift priority, for example a *Layer 2* shift overrides a *Layer 1* shift.
+In this case, users in a *Layer 1* shift will not be notified during the time overlapping with a shift in *Layer 2*.
+
 ### Add an on-call schedule to escalation chains
 
 Now that you’ve created your schedule, it must be referenced in the steps of an escalation chain for on-call users