Hi,
The suggestion to be used when the SM is in router NAT mode (typicial scenario for internet customer) but the ability to bridge an VLAN between WAN (wireless) and LAN is useful when having voip and specially FAX IP (T.38) which doesn't like the NAT or SIP helpers.
In some cases I have to put the epmp sm in bridge mode, then the ATA then a wan nat router.
In ubiquiti and albentia it is possible to have the untagged lan for internet devices and pass a vlan directly from the wan for the ATA voip service
regards
Antonio