Your machine will read a certain type of format. Your embroidery software will produce another type of format. If you were to create a text file in your software (a name for example), you'll want to save it as a software file and a machine file. I run Tajima machines and use Pulse software so I produce a PXF file in Pulse (the file type my level of Pulse creates) and also save the file as a DST (the file type my machines read). If you plan to outsource digitizing (a great idea) You might want to look for a digitizer who can send you a software file rather than a machine file. This will allow you to make minor changes yourself. If you find a good digitizer you won't need this capability often but its very helpful if you need a simple small edit late at night and the job is due the following morning.
How the fabric affects digitizing has to do with how the stitches push and pull and sink in depending on the fabric type. For example, stitches tend to lie on the top of tightly woven fabric like woven shirts and sink in to polar fleece. The digitizer must know the fabric type so he/she can adjust the stitches to accommodate the fabric. Mention the fabric type when you send out jobs. You don't have to be be too specific. Just say pigue, jersey knit, 100% polyester, woven or whatever you need.