The primary field is always the first field in any record, and cannot be deleted, moved, or hidden. The primary field is meant to be a short, unique representation of each record in your table. The primary field is required so that it can be used as a brief description of a record in other parts of the UI. For example, if you look at the record in the table, or if you link a record to another table, we show the contents of the primary field in the top left of the card.

When you expand a card, the primary field will always be at the top of the expanded card.

The primary field is most often a text type field, but it can also support most other field types, including date, number, and auto number. However, the primary field will  not support attachment, checkbox, single select, multiple select, rollup, lookup, and count type fields, or links to other records.