I’m not sure. I think mp4.
If I go into the System tab, within the Notes section it reads:
“Use this setting to create an MP4 (AVC/AAC) file for progressive internet downloads.”
Next to Format, it reads:
“Audio: 192 Kbps, 48,000 Hz, 16 Bit, Stereo, AAC
Video: 23.976 fps, 640×360 Progressive, YUV, 12 Mbps
Pixel Aspect Ratio: 1.000”
Does that help?