The best way to identify the correct capacitor is using the correct nomenclature from the HVAC equipment nameplate. Getting the correct factory part will keep your UL listing intact and you will know your equipment is operating at factory specs. Sometimes the tag is not legible on older units. I have added an image showing the tag on 2 different brands of capacitors. The information required for replacement is the uf rating and the VAC rating. As you can see there are 2 different ways to list the information used by 2 different manufacturers. In the image the capacitor on the left lists the uf ratings as 50/5 uf and 440 VAC. The capacitor on the right lists the same ratings as 50 uf     5 uf and 440VAC.

The capacitors we show in the picture are manufactured for Carrier Corporation with a Carrier part number on each one. The capacitor on the right has Carrier part number HC98KA051D. The D in this part  number is not used so the actual part number is HC98KA051. This part number has been replaced by Carrier with a new part number. New part number P291-5054RS is the factory replacement for HC98KA051. If you provide the product number from the Carrier equipment you are servicing we have access to a Carrier database that lists the current Carrier part number for all parts used in your equipment. By providing the product number you can be assured you will get a brand new Carrier factory part and your equipment will be operating at its original specs and efficiency.