For a futures instrument that is linked to an underlying cash market it refers to when that cash market trades
This is only applicable to equity futures contracts
which are based on indices of a real underlying stock exchange that trades only for a few hours each week day.
Eg the e-mini S&P500 futures contract
at the CME
(ES) trades as a futures contract from 6pm USA ET the day before until 5.00pm ie for 23 hours. Whereas the underlying stock exchange index it is based on only trades from 9.30am until 4.0pm weekdays. So 9.30am is the Cash Open and 4.0pm is the Cash Close.
Note Cash Open and Cash Close do not mean exactly the same as Regular Trading Hours (RTH
). For instance oil as a futures contract at the CME also trades from 6pm the day before until 5pm on the current day. There is no underlying exchange/index that trades seperate hours ie no Cash Open or Cash Close, but there are recognised Regular Trading Hours stemming back to times gone by when the oil futures contract was traded in the pit
between 9am and 2.30pm. So although the oil pit does not exist today, most professional traders of oil still conduct most of their business between 9am and 2.30pm and hence this period is known as the Regular Trading Hours (RTH).