[css-variables] Which value is substituted?

Hi, all.  Sorry if this has already been answered, but it wasn't clear to
me from reading the spec.  When using a custom property reference (i.e.
"var(foo)"), the spec says to substitute the "value" of the associated
variable (relevant quotes are "... the variable's value is substituted as
normal" and "A variable is substituted for its value in the property value
at computed-value time.").  What's not clear to me here is what "value"
actually means in this context.  The only reasonable candidates I can think
of are the specified value and the computed value, but I think it's
important which one of these is meant, and I don't think it's clear in the
spec as written.

Thanks,
Tom