Programing Help.

I'm trying to get a handle on programing my machine, it has a Fanuc OI-Mate MB controller and this is my first CNC.
I have been looking through a number of the programs that were left on the machine from the last owner
and I see that every time the program comes to a tool change it has the following bit in the program (the number change for what ever tool it is).

N44G91G28Z0.
N45G91G28X0.Y0.
N905M1
N46T05M6
( TOOL 5 - 14MM DRILL/16MM C/BORE )

So I have 2 questions, question 1 what is the line N905M1 for? and the second question is, do I have to put G91 G28 Z0 and the G91 G28 X0.Y0. before every tool change?

Rick