flat2lla parameters?

I was wondering why we need these paramters:

llo - Reference location, in degrees, of latitude and longitude, for the origin of the estimation and the origin of the flat Earth coordinate system.

psio - Angular direction of flat Earth x-axis (degrees clockwise from north), which is the angle in degrees used for converting flat Earth x and y coordinates to North and East coordinates.

href - Reference height from the surface of the Earth to the flat Earth frame with regard to the flat Earth frame, in meters.

1) What is the llo - the reference location and why is it in degrees of lat, long? If we only have the flat earth coordinates x, y, z in METERS, then how are we supposed to obtain a reference position in lat, long? I am assuming that the reference location is the starting position, but how do we convert from METERS to LAT, LONG in the first place?

2) What is psio and how do we calculate it?

3) What is href and how do we calculate it?

Please any advice is greatly appreciated.

Thank you!

[Oguzhan Cam]

may be the following link helps,

https://www.mathworks.com/help/aeroblks/flatearthtolla.html