Oh , i don’t come here for a couple of days . Thanx Zen? , actually , I rarely see mel reference , I will have a close look at it .Well , I should be responsilbe for the coordinator system . But I assume that if they should use +Y “pointing” up but not “=” up . What if “up” is a varible which actually holds a value?:D: . I post this tip just want to find out that if the “+Y = up” is widely used in MXS. They assume that I should finish the test in a whole day , and i just finished it in ten minmutes . The test paper prevent me from seeing the real problem of what it really is . Actually ,recently I do have some problems , for I must use the email to communitcates with my friends . Althought I love to do this job to design MXS , I have a deep anxiety that if I do not know the actually problems . Actually , I do not have much experence in game industry . I just learn max myself .
To be honest ， If that is science , all thoeries/formulas are thoeries/formulas , only if they are under certain context/conditions . If not , anything can happen . I do not kown how widely this description has been used . I insisted on my idear that do use “+Y = up” under certain context .
Oh , I just a learner . I just make mistakes every day in every aspects 
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Perhaps , actually , I may still be going to make mistakes for a couple of years:P
I really expected the thread I rised is just limited as a discussion about technical problems .And pretty glad to have a hot discussion.
Based on this . See the following :
“The Maya mel reference/ help files explicitly warn against comparing the result of any floating point operation to 0, for the same reason.”
Models which begin in the top view in max , it’s up axis is the perfect world-up vector [0,0,1] . I have test it for more than once . No need to worried about the comparsion . Models begin in other viewports , do not have this advantage.
Any comments , disagreements , qustions , are still welcome and appreciated