UPDATE 11/17/2017:

The Segwit2x hard fork was called off and did not result in a chain split.

Bitcoin Segwit2x (AKA NYA agreement) is a hard fork scheduled for block 494,784 to increase the Bitcoin block size from 1 MB to 2 MB. Currently 83% (as of block 493,413) of mined blocks are signalling support of the Segwit2x hard fork.

Bittrex did not sign the NYA agreement and endeavors to be neutral for any fork. Our priority is the safety of customer funds.

Bitcoin Segwit2x is considered a "contentious" hard fork which has divided the Bitcoin community and may result in a chain split. There is also no replay protection present so Bittrex must undertake extra measures to ensure that in a chain split, user coins are preserved on both chains. There may be delays in Bitcoin deposits and withdrawals for some time due to these extra measures.

The "BTC" ticker will remain the Bitcoin Core chain before the hard fork block. Bittrex will observe the Bitcoin network for a period of 24 to 48 hours to determine if a chain split has occurred and the outcome.

In the event of a chain split, "BTC" will remain the existing Bitcoin chain with 1 MB blocks until the industry and ecosystem demonstrates a clear chain preference for Bitcoin. Bittrex will take all reasonable steps to ensure that customer funds can be preserved on both chains. Bittrex will make all reasonable attempts to credit our users chain split tokens. We will not recover incorrectly made cross chain deposits under any circumstance due to the lack of replay protection.

If it is unclear which chain is the preferred and dominant chain, we may:

Open up markets for B2X (Bitcoin Segwit2x chain with 2 MB blocks).

Extend the observation period.

If you have a Bitcoin (BTC) balance on Bittrex during the Segwit2x hard fork block 494,784 occurring in mid-November, you will be additionally credited the equivalent amount of chain split tokens on a 1:1 basis. i.e. 1 BTC on Bittrex held during the on-exchange snapshot will get you 1 chain split token. BTC held on orders will also be credited. Only the BTC held on your account is eligible for chain split tokens.

Bittrex will close deposits and withdrawals of Bitcoin (including 2nd layer protocol XCP/OMNI and associated assets) approximately 24 hours prior at block 494,640 to the Segwit2x fork block 494,784 occurring some time in mid-November. If any BTC deposits you had sent to Bittrex are not settled (e.g. marked as pending) by block 494,640 you will not be credited with chain split tokens. The wallet will remain closed until the hard fork block is complete and it is deemed safe to open. Trading will remain active during this time.

Further details:

BTC1 Segwit2x client GitHub

New York Agreement