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SIF refers to the Resolution of some video files. SIF resolutions are 352x240 for NTSC or 352x288 for PAL video. The name is derived from a video conferencing standard known as CIF, which isn't tied to a consumer video system, but rather has a fixed resolution of 352x288.
SIF resolutions are used for VCD and occasionally for DVD-Video, although this is rare in most parts of the world.