It's just the selective recognition (saññā) connected with sensual desire (kāma), as explained in the Atthakatha:ignobleone wrote: If sense-based perception exists in the 1st jhana, then you might have different interpretation of kāmasaññā. What do you think kāmasaññā means?
Kāmapaṭisaṃyuttā saññā kāmasaññā. Byāpādapaṭisaṃyuttā saññā byāpādasaññā. Vihiṃsāpaṭisaṃyuttā saññā vihiṃsāsaññā.