If the substitution is on a users profile (e.g. Custom HTML field) the target user is typically the profile user. In that case, yes. If the substitution is outside the profile (e.g. CB Content Module) the target user is typically the viewing user. In this case, no. This is due to there being no user to substitution off of. To avoid this you can specify the user parameter on the substitution to substitute in a tab or field from a specific user if you wanted. See the below substitution tutorial for further usage information.
www.joomlapolis.com/support/tutorials/107-use-cases/18353-using-substitutions-throughout-cb
In your case you need to have an IF substitution to see if someone is a guest. If they are substitute in the field of user 566. If they aren't then substitute in the field normally. Please see the bottom of the substitution tutorial page as there are IF substitution usage examples for checking for guest, etc..