The horn sounds a fifth lower
The horn in F sounds a perfect fifth below what's written. For a concert C, the player reads a G. A piece in C major is written in G major, and one in B♭ major in F major.
Key signature or accidentals
In classical repertoire, horn parts were often written without a key signature, with every accidental written in. Today it's usual to write the key signature, which is what the tool does.
Double horn
The F/B♭ double horn always reads the part in F; switching sides is up to the player, not the part.
Frequently asked questions
Does it work for E♭ alto horn?
Choose “E♭ · alto sax”: it transposes the same way.
Is the part moved to treble clef?
Yes. If you want the lowest notes in bass clef, adjust them afterwards in your notation program.