basic question about variable assign

variables { 
  { 
    expression "expr {[mcget {session.logon.last.password}]}" 
    varname session.logon.last.password2 
  }
}

In your example, the expr is actually not needed. The above will produce the same result as in the config:

variables { 
  { 
    expression "mcget {session.logon.last.password}" 
    varname session.logon.last.password2 
  }
}

However, you can evaluate any tcl expression using "expr". For example:

variables {
  {
    expression "expr {[string tolower [mcget {session.logon.last.username}]]}"
    varname session.custom.user
  }
}