For making editorial decisions, logging and referencing, it is useful to have a viewing copy, in any format, which has 'Burnt in Time Code'. A "BITC" copy. Pronounced Bit C.
A BITC copy has the Time code, a tape/time reference, visible or "burnt in" on the screen. In effect every 25th of a second, every frame, has a unique number.
Time code is always written in this format 00:00:00:00
That is Hours, Minutes, Seconds, Minute HH:MM:SS:FF
This enables you to give specific and exact instructions on what you want, down to an exact frame, a 25th of a second. However unless you want a specific frame, for example for use on a website or on a dvd box cover, quoting the hours, minutes and approximate seconds is normally sufficient detail.