When providing user-defined unique IDs (UIDs) for content types and entries, there are certain keywords that you cannot include in the UIDs as follows.
Restricted keywords for uids for Content Type
The following set of keywords cannot be used as the UID of a Content Type:
Restricted keywords for uids for Fields
The following set of keywords cannot be used as the UID of a Field:
Warning: Fields cannot have UID ending with “_ids.”