“Felt happiness” (vedayita sukha) means the vedanā that arises dependent on either the five cords of sense pleasure (kāmaguṇa) or the eight attainments (samāpatti). But this happiness is not unqualifiedly happy for it is still subject to saṅkhāara-dukkha. (“Whatsoever is felt, all that is included in dukkha” — MN. iii. 207).
“Non-felt happiness” (avedayita sukha) is not any kind of vedanā, but rather is a term for nirodha. Being free even of saṅkhāra-dukkha this is an unqualifiedly happy state.
Khandhānaṃ rāsaṭṭhaṃ, āyatanānaṃ āyatanaṭṭhaṃ,
Dhātūnaṃ suññaṭṭhaṃ, indriyānaṃ adhipatiyaṭṭhaṃ,
Saccānaṃ tathaṭṭhaṃ aviditaṃ karotītipi ‘avijjā’.
It prevents knowing the meaning of heap in the aggregates, the meaning of actuating in the sense-bases, the meaning of voidness in the elements, the meaning of predominance in the faculties, and the meaning of suchness in the truths, thus it is called ‘ignorance’.
(Visuddhimagga XVII. 43)