The primaryKey value should be the name of any Column in the Blotter which you can guarantee to have unique values.
Because the Adaptable Blotter is fully data agnostic and doesn't know anything about the underlying data it holds, the only way that it can identify a particular cell is as a combination of Primary Key value and Column value (e.g. the user has edited in the 'Bid' column in the row where the value in the Primary Key Column is 15).
Given the importance of the Primary Key the Adaptable Blotter will warn you at start-up if you have provided a value for this property which does not match a column in the grid.
If you want to remove this warning set the showMissingPrimaryKeyWarning property to be false in General Options section (the default is true).
If a user attempts to enter a duplicate value into the Primary Key column an error alert is triggered and displayed.
If you want to remove this warning set the preventDuplicatePrimaryKeyValues property to be false in General Options (the default is true).