Thank you very much Peter for bearing my broken knowledge and giving me guidance in clearing out my various doubts throughout.
One of the photo which I shown you earlier also, I am attaching again:
I got a comment from person who set that boundary condition in Femap. He commented:
Question I asked: So why in global analysis rotation about Z is free ?
His reply: If you are adding SWBM(Still water Bending Moment) then ur case( in photo) is global analysis then rotation about longitudinal axis in ur project(in photo) Z axis is needed to be free. Because SWBM(Still water Bending Moment) and wave bending moment is maximum in head sea condition and in that condition rotation about longitudinal axis is important.
Question again I asked: But in this photo, rotation about transverse axis is free, not longitudinal I guess.
He replied:Rotation in longitudinal axis means u have to free transverse axis.

What he is trying to say, I don't understand.
Please it's, my last question, I already appreciate your all support in last all comments.
And sure I will go through this pdf you attached.
Appreciate your support and help Peter­ƒæì