W Markers are bypass orders.

I understand it to be part of a larger set trade (or other variable) in order to accomodate the execution of that trade.

"They allow a Participating Organization (PO) to enter orders on the Exchange using a bypass order marker that will ensure that the PO's order will only execute against the visible portion of orders on the Exchange. This will allow POs to fulfil their "best price" obligations while giving standing only to the visible portion of orders on the Exchange. "

Additional details: https://www.osc.gov.on.ca/en/Marketplaces_xrr-tse_20080718_amd-bypass-orders.jsp