Concert Pitch

See https://musescore.org/en/handbook/3/concert-pitch and https://musescore.org/en/handbook/3/transposition#concert-pitch

Some instruments are transposing: e.g. alto saxophone in E flat, trumpet or clarinet in B flat, horn in F and many others.
Their notation is therefore different from the notation of non-transposing instruments such as piano, flute, trombone and others.
This means that if you have a C4 note on the piano staff, for example, and the same pitch C4 is to be played by the saxophone, trumpet or horn, they will be notated on a different staff. See here, they all sound like C4:

(Transposing notation)
And these instruments have a different key signature due to their transposing nature.

If you activate the concert pitch button, you will see the notes of all instruments on the C4 line. This is not useful for a musician who is supposed to play the notes.

(Concert pitch notation)

But I think it's important for a composer who wants to write the right chords for the band. So if his score is to play a C chord, for example, he needs to have a sounding C4, an E4 and a G4. But the players need to see the transposing notation for these instruments. So he switches to concert pitch and distributes C4, E4 and G4 to his various instruments. Switching off the concert pitch mode then shows how the players should see the notes on the sheet so that they also play the correct pitches for the chord.