hey, i just realli realli dont understand the restrictions for parameters in trig

so for this question

4. x = 2cos theta , y = 2sin theta

and 0<theta<2 pi

the equation is x squared +y squared = 4

i understand that

but then the restriction is -2>x>2 only

i get that by drawing the cos graph there is a restriction on the y axis as the cos graph is between 2 and -2, but how du u no that the restriction is on the y axis and not on the x axis?

AND why isnt there a restriction for the sin graph, ?? why isnt therre a restriction placed with y ?

im SO LOST !!!

any helpp apreciated !!