What is ExtraNonce2? I'm reading about the Stratum protocol where miners contribute to pools.  Specifically there is a "mining.submit" method -- https://en.bitcoin.it/wiki/Stratum_mining_protocol, in which I miner will submit a share for consideration.  The details on this doc are a little sparse.  Specifically, what is the "ExtraNonce2" value they describe as being submitted back to the pool?
The extra nonce is a value in the coinbase transaction which miners will increment in addition to incrementing the nonce in the block header so that there are more possible hashes that can be tried with a given set of transactions.

The ExtraNonce2 is specifically the part of the extra nonce that the miner is changing while performing work for the pool. There is also an ExtraNonce1 which the pool provides to the miner. That ExtraNonce1 is typically used as an identifier.