You need both. Plate for your car; placard when you’re in someone else’s

I have plate on my car and placard to take when with someone else driving their car.

I have both because when I go for an epidural I can’t drive home so I use the placard for the person that’s driving me

I prefer the placard because if I ride someplace in someone else's vehicle I can take my placard with me. If I only have the plate than I can't use it in someone else's vehicle.

placard so I can take with me when someone else is driving.