Extra heavy duty rubber cap rated for a long service life with heated pressurized coolant. Don’t trust cheap parts store vacuum caps that will fail and leave you stranded!
This cap is needed when installing a 2ZZ engine from a Celica or Lotus in the MR2 Spyder. You need two of them to fit at locations #2 and #3 in the diagram. If your 2ZZ came from a Corolla or Matrix model then this cap does fit at location #2 but your engine probably already has one installed. Corolla/Matrix engines do not require a cap at location #3.