Thomas Allan Palmer:
If you are referring to the Level Dimension Tool, you can select just the text of a placed marker, open its settings, then select Custom Text and insert Autotext for Project Zero or Sea Level. You cannot select the reference level before placing; if you search the forum you will find other posts about this and maybe a poll.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David