Yes there isn't much documentation specifically on this! This may help. I find "strict matching" a bit misleading as I'd expect that to mean the MAP won't join another BGN but that is not the case.
http://www.cisco.com/c/en/us/td/docs/wireless/technology/mesh/8-1/design/guide/b_mesh_81/Connecting_the_Cisco_1500_Series_Mesh_Access_Points_to_the___.pdf
Configuring Bridge Group Names Bridge group names (BGNs) control the association of mesh access points. BGNs can logically group radios to avoid two networks on the same channel from communicating with each other. The setting is also useful if you have more than one RAP in your network in the same sector (area). BGN is a string of 10 characters maximum.
A BGN of NULL VALUE is assigned by default by manufacturing. Although not visible to you, it allows a mesh access point to join the network prior to your assignment of your network-specific BGN.
If you have two RAPs in your network in the same sector (for more capacity), we recommend that you configure the two RAPs with the same BGN, but on different channels.
When Strict Match BGN is enabled on the mesh AP, it will scan ten times to find the matched BGN parent. After ten scans, if the AP does not find the parent with matched BGN, it will connect to the non-matched BGN and maintain the connection for 15 minutes. After 15 minutes the AP will again scan ten times and this cycle continues. The default BGN functionalities remain the same when Strict Match BGN is enabled.
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