[R] what does the within component of varcomp (ape library) output indicate?

[Sharada Ramadass]

I am trying to use varcomp to obtain the variance partitioning across
different nested levels of random effects (say x,y and z). I get the
three variance components (for each of my along with an additional one
called 'within' from varcomp output. I am using the 'scale total
variance to 1' option and though the within component is small, it
does form a part of what explains the complete variance. I am unable
to understand what variance this 'within' defines.

Can someone kindly explain or provide a reference?

Thanks,
Sharada