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The following is for OpenVMS. (I don't know if it also holds true for Tru64 Unix, although I strongly suspect it does.)
With the EVAs, the "OS Unit ID" directly maps to the Device name. For example, an OS Unit ID of 42 becomes $1$DGA42, and an OS Unit ID of 5678 becomes $1$DGA5678.
For the HSGs, the "IDENTIFIER" is the name. Again, "IDENTIFIER = 42" from a "Show unit full" command becomes $1$DGA42.
I have no experience with the MSAs, although given their history, I assume they have a LUN parameter that works exactly like the EVA's OS Unit ID and the HSG's IDENTIFIER.
The above is why it is critical to ensure multiple LUNs do not have the same OS Unit ID. At best, VMS would get very confused and simply stop accessing the disks in question if it 'sees' disks with different LUN WWIDs and the same OS Unit ID.
XPs, at least all the XP1024s I have seen, are a completely different creature (which is no surprise, given that the EVA, HSG and MSA all came from DEC/Compaq, and the XP has its roots in 'classic' HP). On the XP, LUNs are designated by their starting Controller Unit and LDEV numbers (CU:LDEV), which are written in hexadecimal notation. Combined together and converted to decimal, the CU:LDEV again directly maps to the VMS Device name. The LDEV is the low unsigned byte, and the CU number is the high unsigned byte.
Keeping the same examples as before, a CU:LDEV of 0:2a becomes $1$DGA42 (hex 02a = dec 42), and 16:2e becomes $1$DGA5678 (hex 162e = dec 5678).
Remember, the XP's CU and LDEV numbers are in hex.
Note: While I am an HPE Employee, all of my comments (whether noted or not), are my own and are not any official representation of the company
With the EVAs, the "OS Unit ID" directly maps to the Device name. For example, an OS Unit ID of 42 becomes $1$DGA42, and an OS Unit ID of 5678 becomes $1$DGA5678.
For the HSGs, the "IDENTIFIER" is the name. Again, "IDENTIFIER = 42" from a "Show unit full" command becomes $1$DGA42.
I have no experience with the MSAs, although given their history, I assume they have a LUN parameter that works exactly like the EVA's OS Unit ID and the HSG's IDENTIFIER.
The above is why it is critical to ensure multiple LUNs do not have the same OS Unit ID. At best, VMS would get very confused and simply stop accessing the disks in question if it 'sees' disks with different LUN WWIDs and the same OS Unit ID.
XPs, at least all the XP1024s I have seen, are a completely different creature (which is no surprise, given that the EVA, HSG and MSA all came from DEC/Compaq, and the XP has its roots in 'classic' HP). On the XP, LUNs are designated by their starting Controller Unit and LDEV numbers (CU:LDEV), which are written in hexadecimal notation. Combined together and converted to decimal, the CU:LDEV again directly maps to the VMS Device name. The LDEV is the low unsigned byte, and the CU number is the high unsigned byte.
Keeping the same examples as before, a CU:LDEV of 0:2a becomes $1$DGA42 (hex 02a = dec 42), and 16:2e becomes $1$DGA5678 (hex 162e = dec 5678).
Remember, the XP's CU and LDEV numbers are in hex.
Note: While I am an HPE Employee, all of my comments (whether noted or not), are my own and are not any official representation of the company
