Flag sequence

Flag sequence: In data transmission or processing, a sequence of bits used to delimit, i.e. mark (computing), the beginning and end of a frame.

Note 1: An 8-bit sequence is usually used as the flag sequence; for example, the 8-bit flag sequence 01111110.

Note 2: Flag sequences are used in bit-oriented protocols, such as Advanced Data Communication Control Procedures (ADCCP), Synchronous Data Link Control (SDLC), and High-Level Data Link Control (HDLC).

Source: from Federal Standard 1037C