Status display as dropdown/text field vs lookup

Document ID : KB000097567
Last Modified Date : 22/05/2018
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On the Status display for a ticket, what controls its display as a text field (lookup) vs. a dropdown?  
For the status field:

Dropdowns will usually present if there is a valid status transition from the status that has been selected.  However, lookups will present on the status field if there are more than 9 possible entries or if multi-tenancy is active. (see KB000019316)