Can not use GetAccountInformation API to get the useseat

Hi everyone,

When I use /v2.1/accounts/{accountId} this api to get seatInUse field, some account can return number, some are return null.
this account seatInUse is null

this is normally
 

May I ask what this field is related to? And can you analyze for me under what circumstances there will be a quantity of seatInUse?

Thanks a lot!