[M4IF Technotes] meaning of alternate_vertical_scan_flag!

[gaodd]

"alternate_vertical_scan_flag" is a flag decoded from VOP Header, when set to '1', it indicates the use of alternate vertical scan for interlaced VOPs. but i want to know: when set to '0', which scan method was used for interlaced VOPs? Zigzag or alternate-horizontal scan?
thanks for help!
Best Regards
Gaodd 
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