VANC stands for Vertical Ancillary Data Space. This is part of the transmission signal for digital video that is not displayed on the screen as video frames. It is available as a means of carrying data other than the image.

It is described in the SMPTE

Society of Motion Picture and Television Engineers

The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers (SMPTE , founded in 1916 as the Society of Motion Picture Engineers or SMPE, is an international professional association, based in...

 specification SMPTE-334M "Vertical Ancillary Data Mapping for Bit-Serial Interface".
In the PAL

PAL

PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...

 and NTSC

NTSC

NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...

 analog television systems the Vertical blanking interval

Vertical blanking interval

The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...

(or VBI) is similarly used to carry non-display data.