The BitMEX .BCHXBT30M Index is a 30-minute TWAP of the Bitcoin Cash price at Poloniex ending at 12:00 UTC from the .BCHXBT Index. These values are used for settlements. The TWAP is calculated using values from the BitMEX .BCHXBT index.
This index is composite, which means the price is built from multiple sources. See the "Composite Index Breakdown" below for information on the constituents. If an exchange is to lose service and no trades are printed for over 15 minutes, BitMEX services may automatically remove that constituent from the index until trading resumes.