[Mp4-tech] [H.264]Question about the UnusedShortTermFrameNum variable

[Tan Rui -- Huawei]

Hi, All experts,
When I read the protocol, I find it is difficult to understand the description about the UnusedShortTermFrameNum variable.
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In the protocol, the variable is defined as the following:
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When the value of frame_num is not equal to PrevRefFrameNum, the following applies.
¨C There shall not be any previous field or frame in decoding order that is currently marked as "used for short-term reference" that has a value of frame_num equal to any value taken on by the variable UnusedShortTermFrameNum in the following: [[ I can not understand the sentence, because there are two 'that..' clauses. What is the main idea?]] 
UnusedShortTermFrameNum = ( PrevRefFrameNum + 1 ) % MaxFrameNum 
while( UnusedShortTermFrameNum != frame_num )                                   ( 7-21)
    UnusedShortTermFrameNum = ( UnusedShortTermFrameNum + 1 ) % MaxFrameNum
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Bay the way, Can some one explain that when will the PrevFrameNum variable be equal to the frame_num variable?   
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