You need to do your homework. There is quite a lot of variety: age restricted; sheltered and very sheltered. And probably more. I am not an expert, simply someone who climbed a steep learning curve fast.
Age restricted tend to be the big builders, McCarthy and Stone etc. Usually very little care/support is available and indeed the one I enquired about would not allow anyone below the set age to live there. No good if you needed temporary live in care following a hospital discharge.
Extra care usually means 24 hour warden cover. DM lives in an extra care flat and it is fab. Reception from 7.00am to 10.00pm, scope to eat in the 'restaurant', laundry service, weekly clean, maintained gardens, handyman employed during the week, etc. Service charges are about £600pm but so much cheaper than if she were in a home. (She has lost most of her memory.) And much nicer.
Standard sheltered, I think, means without the 24 hour warden.
You need to be very careful though. Some leases are very restrictive, making flats hard to sell. And if you can't sell you are stuck paying a high service charge. Some national management companies have a poor reputation. In contrast flats in my mums block often sell at a premium. I guess you could get a rough idea of popularity by doing an advanced search on Right Move and ticking the age restricted box. Blocks that are cheap probably are for a reason.
I ended up using a local solicitor who has built some expertise in sheltered housing leases. PM if you need a name.