Allied Telesis AT-S63 - Manual (Page 209)

AT-S63 Management Software Features Guide
Section IV: SNMPv3 209
After you specify a MIB subtree view you have the option of further
restricting a view by defining a subtree mask. The relationship between a
MIB subtree view and a subtree mask is analogous to the relationship
between an IP address and a subnet mask. The switch uses the subnet
mask to determine which portion of an IP address represents the network
address and which portion represents the node address. In a similar way,
the subtree mask further refines the subtree view and enables you to
restrict a MIB view to a specific row of the OID MIB table. You need a
thorough understanding of the OID MIB table to define a subtree mask.