Song and Session Start

[mnstudio]

I haven't even understood the difference between Song Start and Session Start in Pro Tools. And what is it for.....
For which reason I could change one or the other...?
For example I have a song with Song start moved (I don't know how I got it....) and if I export its MIDI file and re-import it (with tempo map) on another empty song, the click doesn't add up...
So I tried to bring back my song start to the start of the timeline, but it moves ALL my tracks making big confusion.
I tried to move song start without moving the tracks, maybe with "Events > Move Song Start > Realign Song Start only" but this is greyed out.
I come from Logic where there wasn't such a distinction
Any clarification would be appreciated

I attach my song start movement

[daeron80]

Session Start defines how timecode maps to sample 0 on the timeline. Song Start defines where Bar 1, beat 1 falls on the timeline.

You don't need to worry about Session Start if you're not working to video. Make a session, set your tempo and meter, decide how much count-in you want. If you want a 2-bar count-in, move the Song Start two bars later so Session Start is -2 bars.

[skylar_battles]

If conductor is on and you try to move the song start marker it will move tick based tracks unless you hold control shift which prevents that from occurring. You can also do this in the time operations dialogue which options that specify what behavior will occur. Also, return will bring you to session start while control return will bring you to song start.

[mnstudio]

Thank you very much daeron80 and skylar_battles . Both your answers are so clear and useful to me