Sundry debtors are clients/customers that rarely make a purchase on credit. They will show up as a single line on your Trade Debtors report.
For example say your business was a Corporate Travel Agent. You may have clients who book hundreds of flights and hotel stays with you a month. These clients would all have their own “account” on your finance system. A sundry debtor may only book one hotel room in a year , so you might have their purchase added to the Sundry Debtor account.
Many businesses have finance systems that make it unusual to have a Sundry Debtors account. This is due to how easy it is to add them onto your finance system and how useful it is for analyzing your Sales Ledger.