I used to run that machine many moons ago. I run swiss now just not that model.
Swiss are a"different" breed. You will need to change the way you think. These machines work GREAT but you need to accept the way the machine thinks and work with it that way or it will drive you crazy.
The machine works like a "cam" machine. All the tools and axis' are put at a "Starting Position" at the start of the program and return there at the end of the program. This is done using the machine coordinates and work shifts
(G50).
The "G300" line you mention is how this machine goes to "start Position". This is the part length and tool "geometry" added together. The is done with the collet open so the stock does not move during the motion. The barfeeder holds it in place.
The "G50" line sets the work coordinate system for the part before any movement of a tool or axis.
The setting of tools for that machine was done with a "pesetter". The whole thing works from a theoretical zero position.
The "Z" axis is the opposite of a cnc lathe. This is because the stock is moving across the tool not the tool across the stock.
This is just a very brief description, you should seek better help from Rem Sales to be more successful.
Good luck! Welcome to the "DARKSIDE"