each long-arm quilter has their own (policies/specifications) so you need to contact the long arm(er) and ask ...but when long-arm quilting each part is loaded separately...the backing, then the top-then the batting floating between- the sandwich is not basted together first-the parts are loaded onto roller bars.
normally your long armer will want the backing and batting a few inches larger than the top all the way around- the top and backing will need to be pressed nicely- squared up, loose threads trimmed- if there are alot of seams along the outside of the quilt sometimes its a good idea to run a (stay-stitch) all the way around the quilt to keep those seams from pulling apart when loaded. also, (i always ask) that if a person is piecing their backing they have the seam horizontal across the backing- not vertical...when it is horizontal it lays nice & flat, smooth along the bar when loaded- when it is vertical is builds up, 4 layers at a time with each turn of the roller---so you have large bulk in the center and 'floppy' edges...makes it more difficult to keep everything smooth as it is quilted---but you need to talk to your quilter to see how they want things.
you also may want to check out a long arming video (handiquilter has some good ones-among many others) so you can see just how it looks loaded & how it's done- then it may make more sense to you.