OEM parts are Original Equipment Manufacturer parts.This means it is a vehicle part made by the car manufacturer. OEM parts tend to be more expensive and normally a perfect fit.
Aftermarket parts on the other hand are produced by a different company than the car maker. While these parts are usually made with the same materials and made to serve the same purpose,they may have slight differences in how they look and feel and may not always be a perfect fit.
When the insurance company writes or approves the estimate, they determine whether or not to use OEM or aftermarket parts.
We like to pre-order all the parts so your vehicle is not just sitting here waiting to be repaired. As well as some parts might be unavailable or on back order. Instead of extending the completion date and rental it is easier to just wait until all the parts are in before you drop off for repair.