Now I understand what the specification means...
Its referred to FLV files... FLV is supported natively by this version of firmware... internally must be done some conversion before broadcast... and the specification stated the limits of FLV source.
Here is the text translated from Chinese:
"The FLV document please do not have to surpass the following scope,
otherwise will be unable to broadcast. The video frequency size most
greatly supports 400X300, the frame number in 25"
FLV outside of this limits will result in a warning and won't be displayed.