The refers to the usual advice seen on install pages, mentioned in Post #62, to install on top of a specific version of stock firmware.
There is a further comment on this matter as mentioned on XDA (from our very own Sahil_Sonar) Development [ROM][STABLE][tetris][14] /e/OS for CMF Phone 1
Installation steps:
Note: Builds ships with stock firmware, no need to be on specific firmware version.
This quote is from the OP of that thread, one might read the whole thread or ask there for further reassurance before proceeding.
- (To be clear, I do not know the device, but I observe that install is done by a Fastboot flash script, in a similar way to the /e/ install of Fairphones.)