Slippage amount, in points (dollars per share or contract), for each limit order transaction
Any formula specifying dollars per share or contract (points)
Defines the amount of slippage to apply to to each limit order transaction, in price points.
Slippage is calculated and applied to each side of the trade (EntryLimit and ExitLimit) separately.
If LimitSlip is not specified, then the Slippage formula will be applied to limit orders along with other order types.
If you want there to be slippage for market orders but not for limit orders, you must specify LimitSlip: 0.
It makes sense to use non-zero LimitSlip if you use "market if touched" or "not held" (discretionary) limit orders, or to use LimitExtra (with LimitSlip: 0) if you use standard limit orders.
When a strategy uses both EntryStop and EntryLimit (enters positions with a stop-limit order), StopSlip is applied to the entry transaction if specified, otherwise Slippage is used (LimitSlip is ignored in this instance).