Phassa (Contact) comes in the middle of the Paṭicca Samuppāda (PS) process. However, PS processes start with “salāyatana paccayā phasso” or “an āyatana making contact.” That contact is between a rupa and one of the six āyatana (cakkhu, sota, gandha, jivhā, kāya, mano.) At the fundamental level, a rupa makes contact with the mind via one of the five pasāda rupa or directly with the mind (hadaya vatthu.)
Another way to state the difference is to say that when kamma vipāka brings us sense inputs, the sense faculties act as indriya. Following that we MAY deliberately use sense faculties to generate new kamma; then they work as āyatana. […] 12. Thus only Arahants use their sense faculties as indriya ALL THE TIME.
Here is what I don’t understand. Life goes on after attaining Arahanthood. I know that Arahant still senses the world. So, there will be contact between a rupa and one of the six āyatana for Arahant too. But, as you said, “Arahants use their sense faculties as indriya ALL THE TIME”. I understand that the former context is somewhat different from the latter. However, if the two are compatible without contradiction, there must be at least a meaning to explain both in āyatana. I guess that there is a word showing the meaning of salāyatana of Arahant like kiriya(the kamma-free act of Arahant). Is there a word like Salindriya?