How calculated symbols after . or _ ?

Fasmw 1.73
Idea work with vectors. Vec2 or vec3

Code:

mvec option,v { ... }

;in code
mvec add,p1.xyz,p2.xyz 

;generated this code 
movss xmm1,[p1]
addss xmm1,[p2]
movss xmm2,[p1+4]
addss xmm2,[p2+4]
movss xmm3,[p1+8]
addss xmm3,[p2+8]

mvec add,p1.y,p2.y

;generated this code 
movss xmm1,[p1+4]
addss xmm1,[p2+4]

I thinked on variant:
mvec add.xyz,p1,p2
mvec add.xz,p1,p2
mvec add,p1.y,p2.z

Or how calculated how symbols get macro ? Using math for this ?
mvec add,xyz ;get 3 values
mvec add,xz ;get 2 values
mvec add,x ;get 1 value