Here are my diagrams to hopefully help to make sense of the Zoning.
RCF File = Brcd6510_8N_FC1_FC2_FAB1_SW1_D5_RCF_v9.1b
Fabric = 1
Domain IDs = 5 & 7
Image: Zones for Fabric 1: DR Group 1 (MC1)
Image: Zones for Fabric 1: DR Group 2 (MC2)
RCF File = Brcd6510_8N_FC1_FC2_FAB2_SW2_D6_RCF_v9.1b
Fabric = 2
Domain IDs = 6 & 8
Image: Zones for Fabric 2: DR Group 1 (MC1)
Image: Zones for Fabric 2: DR Group 2 (MC2)
(i) Each ATTO has 2 zones. 1 zone for FC1 on in Fabric 1. 1 zone for FC2 in Fabric 2 (e.g 5,8 and 6,8).
(ii) There are 2 initiator groups (INIT_GRP) per ATTO stack (STK_GRP), so that the HBAs on the ONTAP controllers with 4 x HBAs per controller can be split – not only between fabrics, but also – between the “Top” and “Bottom” ATTOs.
(iii) With ATTO 6500N, where we only used one FC port per ATTO, there are no initiator groups. All the HBAs in that fabric are zoned to the ATTO 6500N's FC port.