Individual covariates and missing values

[Ems]

I would like to ask if I have understood the manual correctly:

If I use z-transform standardization is it really so that I only need to put zero to missing values and Mark does the rest or do I have to do some scaling myself?

I read that there would be some problems with standardize individual covariates option if I had two groups and wanted them to have a common intercept. Would there be that kind of problem with two age classes too?