Bitcoin Cash is expected to conduct a hardfork upgrade on 15 November 2018. There are two competing incompatible hardfork upgrade proposals, with the associated clients being Bitcoin ABC and Bitcoin SV. Therefore, there could be a chainsplit; users holding BCH prior to the hardfork could end up with coins on both sides of the split.
On settlement, the BCHZ18 contract will settle at a price on the Bitcoin ABC side of any split and will NOT include the value of Bitcoin SV.
This is consistent with the approach BitMEX took when Bitcoin Cash initially split off from Bitcoin in August 2017. The Bitcoin / USD contracts were settled based on the Bitcoin price at the time and did not reflect the price of Bitcoin Cash.
Impact on the BitMEX .BBCHXBT Index The BitMEX .BBCHXBT Index is currently comprised of the following constituent exchanges:
⅓* Poloniex + ⅓ * Binance + ⅓ * Kraken
At the time of the fork, if the constituent exchanges use the BCH/XBT symbol for the BCHABC chain then BitMEX will keep that symbol as a reference, otherwise BitMEX will refer to the relevant BCHABC/XBT symbol to be used as part of the Index.