sheet properties

[MARCOC]

Hello everyone, can anyone tell me the difference between these two properties? ? ?
I tried to insert them both to are identical, as if I make a new design and choose a4 (he will put all 2 properties a4, if then change and put a2 will become both a2)So what is the difference? ? ?

Thank you very much

 

[MassiVonWeizen]

the name itself says: one represents the size of the sheet format, the other the size of the useful space (the white area to understand) of the model.
obviously when inserting a square the useful area fits and then the two voices correspond; if you do not insert any cartiglio and you set the size of the only useful area the variable size of the model is the only one to operate