In my understanding: Pnd.91 is for reporting salary income (incl. Bonus and comish) paid by your employer and suject to Pnd.1 deductions each month. Pnd.90 is for reporting "other income" which is charged for services and subject to 3% withholding tax. Typically foreigners wouldn't declare Pnd.90 income as such "other income" services (i.e. consulting) would conflict with the Work Permit scope. Hoping this helps.