In Replica Set Priority Documentation of 3.0, at https://docs.mongodb.org/v3.0/tutorial/adjust-replica-set-member-priority/ it is mentioned, "To block a member from seeking election as primary, assign it a priority of 0. Hidden members, delayed members, and arbiters all have priority set to 0.".

However, I have found that in the server with default config, arbiter default priority is set to 1. See following. Either the documentation or the server code needs to be changed. It should be 0 for practical purposes.

{
"_id" : 2,
"host" : "localhost:27002",
"arbiterOnly" : true,
"buildIndexes" : true,
"hidden" : false,
"priority" : 1,
"tags" : {
},
"slaveDelay" : 0,
"votes" : 1
}