This is a link to the official Disney site page about mobility disabilities (there is also a link to that page from the disABILITIES FAQs thread, as Cheshire Figment mentioned).
Once you get to the parks, you will find the individual attractions are marked with icons on the park maps which indicate whether you can stay in a wheelchair/ecv for the whole attraction (a little wheelchair icon), whether you have to switch from an ecv to a wheelchair (a little person getting up from the ecv and going to a wheelchair) or whether you have to get out of the wheelchair and board the ride car (a little person standing up next to a wheelchair). There are signs in front of each ride also with the same icons.
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