Proc Mixed statement

Suppose I have a data set:

Geography Quarter  Y X1 X2
A           1      3  1  4
A           2      2  4  4
A           3      5  5  2
A           4      1  3  4
B           1      3  1  6
B           2      8  6  7
B           3      2  4  1
B           4      9  7  3


What does this statement do?:

proc mixed data = data;
model Y = X1 X2;
run;


Does it fit one model or multiple models? I am trying to understand what proc mixed actually does with this dataset. Would this statement simply fit one model for all data points? And would adding a class statement create a model for A and a model for B?