Hi recently i have try to learn for the expansion of the universe and one of the think which i don't understand is if i have two object (a and b) at fix distance between them 1000 m how this distantes is related with comoving coordinates.

So the formula is : D(a,b)= a(t).x(a,b)

D(a,b) - is the actual distance between a and b
a - is the scale factor
x- is the fix distance

But i don't understand what units has this ''x''. Is it in meters or in grid units. And if ''x'' is in grid units how its converted from meters to grid units?

I've watched this lecture series of Professor Leonard Susskind: https://www.youtube.com/watch?v=P-medYaqVak