Variable load order of CSB variables and cfg files

The _USTN_WORKSPACESROOT variable is populated in the WorkSpace Managed CSB which is loaded after the Predefined CSB and then the WorkSpaceSetup.cfg?

If everything is loaded in order, the WorkSpaceSetup.cfg won't know the value of _USTN_WORKSPACESROOT because it won't load until the WorkSpace Managed CSB is loaded?

Kind of. There are 2 ways to refer to the value of a variable: in parenthesis $(VARIABLE_NAME) and braces ${VARIABLE_NAME}. When parenthesis are used microstation retains the $(VARIABLE) structure and doesn't substitute the value in place until the variable referring to it is used. So as the config files are loaded and microstation has:

_USTN_WORKSPACESROOT : $(_USTN_CONFIGURATION)WorkSpaces/
_USTN_WORKSPACEROOT  : $(_USTN_WORKSPACESROOT)$(_USTN_WORKSPACENAME)/

It'll remember that _USTN_WORKSPACEROOT is $(_USTN_WORKSPACESROOT) and $(_USTN_WORKSPACENAME) but even though _USTN_WORKSPACENAME isn't set yet it keeps processing. Then when MicroStation finally goes to load it looks at the _USTN_WORKSPACEROOT variable and now _USTN_WORKSPACENAME IS defined so it substitutes it in and the variable points to the correct location.

So by using $(VARIABLE_NAME) MicroStation will re-evaluate the variable everytime it's used and use the current values of whatever variables are used in the definition.

(Braces, by the way, mean 'do the substitution right now, and don't remember the source' so _USTN_WORKSPACEROOT = ${_USTN_WORKSPACESROOT}${_USTN_WORKSPACENAME}/ would fail the way you mention)