A certain options contract's market value, one's profit & loss with such a contract, risk control etc. refer to the options contract's Mark Price. A certain options contract's Mark Price from the previous trading day is referred to as its "Pre-mark", used for one's daily profit & loss of that options.
Mark Price at Tiger is as follows:
When the last price lies between the bid and ask, the last price is the Mark Price.
In the absence of a last price or the last price is not between the bid and the ask, the Mark Price will be the mid-price between the bid and the ask (for single-leg options, the mid-price will usually be the average of the bid and the ask).
In the absence of an ask price, the Mark Price will be the higher of the bid and the last.
In the absence of a bid price, the Mark Price will be the lower of the ask and the last.