Accept Unsolicited Notification
Common use case is when Cisco Unity Connection is integrated with Cisco CUCM. A caller leaves a message.
MWI (Message Waiting Indicator) is sent by Cisco Unity Connection to Cisco CUCM using a SIP NOTIFY message.
This SIP NOTIFY message is not part of an early SIP dialog or conversation.
In other words, Cisco CUCM receives a SIP NOTIFY without an early solicitation, neither requested nor solicited.
If the option Accept Unsolicited Notification is not checked. The CUCM sent a SIP Message 403 Forbidden to CUC.
The Accept Replaces Header
Common use case is when integration a cluster of Cisco Meeting Server CallBridges, and the Call Bridge Group feature is enabled. When a conference is alreaty active on one callbridge node, and a second call to the same conference is routed to a second callbridge node. The first callbridge hosting the conference will request to retrieve the original SIP dialog between Cisco CUCM and the second callbridge.
This is done by sending a SIP INVITE with Replace header including the Call ID + TAG FROM + TAG TO that uniquely identify the SIP DIALOG (between CUCM and the second CallBridge).
If the option Accept Replaces Header is not checked. The CUCM sent a SIP Message 403 Forbidden to the first CallBridge.
Demystify and Simplify 