Valid from List & Label 9List & Label's Code 128 always begins with the required subtype and switches to the other subtypes if neccessary. As long as you're only using characters that belong to a certain set normally no switching will be made. As soon as you use a character from a different subset, a subset switch will be made. The character/subtype assignment and switching rules can be found in the Code 128 norm.
Starting with version 13, you may specify the codeset to use. See the "Barcode" chapter of the Designer reference for a description.