From BATS:
The BATS Dark Pool is a separate book which allows matching of dark liquidity based at a midpoint reference price. Orders placed into the BATS Dark Pool do not need to be Large in Scale (LIS). BATS Dark Pool orders only interact with other BATS Dark Pool orders. Quotes for BATS Dark Pool orders are not represented on any market data feed. Matches in the BATS Dark Pool are represented as trades on BATS’ market data feeds, but it is not possible to differentiate between hidden orders executed on the integrated book and on the BATS Dark Pool.
BATS Europe Dark Pool
BATS Europe FIX Specification
Related:
Hide-and-Seek in the Market: Placing and Detecting Hidden Orders