In order for a corset to be considered “custom”, you need to give the corset maker your measurements so that it fits to your body perfectly.
An off the rack (OTK) corset comes as is off the rack.
Sometimes a few measurements can be off, making the corset not fit you quite right, but it will still get the job done.
It’s your choice of either getting an OTK or custom corset, but just know that custom corsets can be much more expensive.
An example of an OTK corset being a bit off, is when I purchase corsets I usually have to lower the bunny ears (loops you pull to tighten the corset), because it doesn’t sit quite right on my waist right out of the box.
One of the brands I need to do this is with Orchard Corset.
Their corsets always sits a bit high on my waist, because my natural waist is, I guess, a bit lower than normal?
Idk.