E4
E5
E6
M1
M2
M3
M4
Those are salary grade. If you search for someone title in the outlook directory.
E5 - project engineer
E6 - senior engineer
M1 - assistant principal engineer
M2 - prinpical engineer

But someone who is M1 can have a job title as "Operations Manager" then this title is given by the division manager cause the division is managed by the division manager. they want to call what. then call what lo. LIkewise in another division someone who is M1 is called "business manager" or someone in M2 is called "technology managers".

Likewise all the E5 - project engineers.
They call themselves as they like. I have seen linkedin resume title like
"Software engineer"
"Software developer"
"Machine engineer"
"Support engineer"
"Project Lead"
"Software Lead"
"Data Scientist"
"Dev Ops"
"SYstem engineer"
"Project Manager" - some even call themselves project manager as E5. =)

ST is really inflated with titles.