You can do complete enumeration...but you risk getting worse main effects in certain cases (compared to Balanced Overlap). That's because if there is a key attribute level that is a must-have, then the respondent only can pick that one concept on the screen (without trading off any other attributes). It means that you'll never learn anything more about that respondent other than the top level thing of importance if that level comes from an attribute where each of the levels is represented exactly once in the choice task.
If you choose "show levels of FIRST ATTRIBUTE no more than once per task" it tries very hard to have no level overlap (no level repeating) for the first attribute. This is important to some researchers if they want each brand to show exactly once within each choice task (imagine 6 brands in the study, 6 concepts per task, and the researcher wants each brand to show exactly one time per task).