Every chord button plays a triad on its own — root, third, fifth. But nothing stops you holding more than one at once, and combinations compose into other chord types rather than needing their own dedicated buttons:

• Two buttons a third apart (skip one — e.g. 1+3, or 4+6) share their middle two notes, and together make a seventh chord. The shared notes don't retrigger — they just keep ringing while the new one joins in.
• Three buttons stacked the same way (e.g. 1+3+5) extend that into a ninth.
• A home-row note that isn't already in the chord shows up in the readout as whatever tension it actually creates — add9, #11, and so on — not just tacked on.

The chord readout always names exactly what's sounding, however you built it. The fastest way in is just to try combinations and read what comes back — that's the whole idea.