https://docs.mongodb.org/manual/reference/replica-configuration/#rsconf.members[n]._id

ReplicaSet configuration "members" "_id" field value range is not documented:
According to https://github.com/mongodb/mongo/blob/r3.2.0/src/mongo/db/repl/member_config.cpp#L225 it's an integer between 0 and 255 included.

Other fields in rsconf.members are fully documented, such as priority: between 0 and 1000.

This undocumented limit was somehow reached by a Chef user (see https://github.com/edelight/chef-mongodb/pull/410).