Common:Many-to-Many Lookup Considerations
From AgileApps Support Wiki
Special considerations for a Lookup on a Many-to-Many Relationship:
- The Lookup automatically becomes a Multi Value Lookup.
- It cannot be made read only.
- Layout Rules cannot test the set of selected values.
- You can specify a record-filter, to limit the number of possible choices. However, post selection JavaScript is not supported here.
- When displayed the chosen values cannot be clicked to visit the corresponding records.