Shimano's eSTePS compatibility is a maze that's claimed the sanity of many a forum member, so let me give you the short version.
The EP801 works with the SC-EM800 (colour display, bar remote), SC-EN600/EN610 (compact colour), and SC-EN500. The older SC-E8000 is
not directly compatible.
One thing worth knowing: the EM800 has Bluetooth built in, the EN600 doesn't. If you want Bluetooth on an EN600 setup, you'll need the SW-EN600L switch to add it.
@Astro66 also flagged that the
EM600L switch won't work with the EM800 display because both carry Bluetooth and the compatibility table has no path between them. Switch and display Bluetooth variants have to match.
Wiring generation matters too. Older EP8 uses EW-SD50 wire; the EP801 and EP6 use EW-SD300. Adapters exist, but if you're upgrading from EP8 to EP801,
@Lummox's experience was that
the entire system needs replacing, wiring and all. Nothing carries over.
Full compatibility charts are at
productinfo.shimano.com if you want to trace a specific combination.
What are you trying to connect? I can be more specific if you give me the motor and display you're working with.