EPH, EPV GPS values

Can somebody tell me what do eph and epv s mean?
Here: https://danielcjacobs.com/uploads/ECHO_Memos/ECHO029_Logged_GPS_Data_Explanation.pdf
They considered as standard deviation.
While here:
SensorGps (UORB message) | PX4 User Guide
Accuracy, but it is not detailed.

Also, since eph and epv are used for failsafe:Parameter Reference | PX4 User Guide
, I want to use them instead of multiplying the measurement resolution with hdop and vdop.
Any feedback on the questions above would be appreciated.

For uBlox the value comes from here:

So it’s UBX-NAV-PVT.hAcc which is described as “Horizontal accuracy estimate” in mm.

See: https://content.u-blox.com/sites/default/files/products/documents/u-blox8-M8_ReceiverDescrProtSpec_UBX-13003221.pdf

Thank you!
My problem with this is that accuracy is not defined here neither. In some places it is listed as standard deviation.

It is a bit confusing. Could you clarify this to me?

Many thanks,

My assumption is that it’s a standard deviation but if you want to be sure you might have to talk to uBlox and ask them.