The difference of ĀnāpānassatiSutta and KāyagatāsatiSutta

1. It doesn't mean "awareness of body sensations" in both sutta.

   It means "The practitioner should meditate ānāpānassati every time, no when&where is an exception (walking, sitting, standing, lying down, bending etc.)"

   "Awareness of sensations" is virtual, IndriyaSaṅvaraSīla, and concentration meditation, Samādhi.

   However, the Anupassanā meditation of sensations is in Vedanā-Cittā-DhammāNupassanāSaṭipaṭṭhāna of MN Mahāsatipaṭṭhānasutta. This section is about the comprehension of nāmas' origin.

2. Buddha taught only the first 1-4/16 steps (Samatha-meditation part) of ānāpānassati to the practitioner who interested in the beginning of the Ānapānassati-meditation, which was talked in the current conversation at the origin of Sutta. Ma. U. Kāyagatāsatisuttaṃ (body/kāya and sati in this sutta refer to kāya-anupassanā-satipaṭṭhāna):

   "Just now, lord, after the meal, on returning from our alms round, we gathered at the meeting hall when this discussion arose: 'Isn't it amazing, friends! Isn't it astounding! — the extent to which mindfulness immersed in the body, when developed & pursued, is said by the Blessed One who knows, who sees — the worthy one, rightly self-awakened — to be of great fruit & great benefit.' This was the discussion that had come to no conclusion when the Blessed One arrived."

   But Buddha taught the full version of ānāpānassati-meditation to Taruṇa-Vipassaka, the practitioners who had already meditated the first 1-4/16 (Samatha-meditation part) of ānāpānassati in the Buddhist Lent of Sutta. Ma. U. Ānāpānasatisuttaṃ, however, it was the end of that Buddhist Lent when they would go to the next step, the other 5-16/16 (vipassanā-meditation part).

   So Buddha decided to remain at Savatthi for another month, pavāranā saṅgha, to turn those Taruṇa-Vipassakas enlighten Nibbāna easier by the other 5-16/16 (vipassanā-meditation part) between that the Buddhist Lent before they transport to the other country after Buddhist Lent because the transportation will make those Taruṇa-Vipassakas meditation dropping down. Then those practitioners enlightened in time, so Buddha concluded the 16 steps Ānāpānassati-meditation of those practitioners as the main content of Sutta. Ma. U. Ānāpānasatisuttaṃ.

   ...

   "Monks, I am content with this practice. I am content at heart with this practice. So arouse even more intense persistence for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. I will remain right here at Savatthi [for another month] through the 'White Water-lily' Month, the fourth month of the rains."

   ...

   Now on that occasion — the Uposatha day of the fifteenth, the full-moon night of the White Water-lily Month, the fourth month of the rains — the Blessed One was seated in the open air surrounded by the community of monks. Surveying the silent community of monks (the silent means all practitioners there have already enlightened and sitting with attainment in Phala-Samāpatti), he addressed them:

   ...

   "In this community of monks there are monks who remain devoted to mindfulness of in-&-out breathing.

   ...(full 16 steps of ānāpanassati meditation)...