Thank you, Gad, for finding the sutta, AN 6.60.
I have re-formatted your reference to AN 6.60, so the original and the translation are side-by-side.
Your question refers to the verse at marker 8.1: “Idha, panāvuso, ekacco puggalo sabbanimittānaṁ amanasikārā animittaṁ cetosamādhiṁ upasampajja viharati” which is translated there as, “Take the case of another person who, not focusing on any signs, enters and remains in the signless immersion of the heart.”
Before that, the sutta explains how Ven. Citta attained anariya versions of the jhanas up to the fourth jhana. Those anariya jhanas are attained by focusing on a nimitta (worldly objects such as breath or kasina.) Then, one can go to higher samapatti states, and the above verse refers to such a samapatti state (corresponding to arupavacara samapatti states).
