Play Bbadd9 one string at a time and you hear Bb, D, F and C. Bb is the root, D the major third, F the fifth and C the ninth. Said aloud, its notes are B flat, D, F and C.
String by string, low to high, it plays Bb, F, Bb, D, F and C. The root, Bb, shows up twice in the main shape. It runs from Bb on string 6 up to C on string 1.
For Bbadd9, your index finger barres fret 6. Your hand covers three frets, from 6 to 8. There is one more position, starting at fret 6.
Bbadd9 and Gm have Bb and D in common. Eb shares one position with this shape.
Bbadd9 belongs to six keys. In Bb major it is the Iadd9, in Eb major the Vadd9 and in F major the IVadd9. The rest appear in the key list.