According to MahasatipatthanaSutta Pali's context, 21 meditations are meditated in four postures, iriyāpatha-pabba, for more proficiency, and the most proficient meditations are the 21 meditations in 7 sub-postures, sampajañña-pabba.
What is SamPajañña?
SamPajañña is the various proficient understanding of the wholesome's object in various perspective.
Saṃ prefix+pa prefix+ñā root+yu root's suffix+ṇya noun's suffix.
Saṃ prefix = sampanna = perfectly complete.
pa prefix = pakāra = many aspects, various perspective.
ñā root = understanding.
yu (na) root's suffix = making ñā to be noun.
ṇya (ññ) noun's suffix = making sampajana word to be state/condition.
Why people can't understand SamPajañña easily?
Because they never memorize kayagatasatisutta pali and mahasatipatthanasutta pali. Every Pali word below is from the sutta directly and I understand it because pali has the very perfect relations in the context.
The kayagatasati pali is very clear and easy--the practitioner pajānāti of breath in sitting meditation until getting proficient level. This develops pajānāti to ñāṇa in sitting posture by the first pabba. Then the practitioner pajānāti breath in every postures by the second pabba, iriyāpatha-pabba. Then the practitioner pajānāti breath in every sub-postures by the third pabba, sampajañña-pabba. The end of all are absorbing Jhana ("Sato ca sampajāno sukhañca kāyena paṭisaṅvedeti" at the end of 21 pabba in KayagatasatiSutta).
MahasatipatthanaSutta is same but it is Vipassana, not samatha. It is the explaining Sutta of the word "Vijjā" of KāyagatāsatiSutta, VipassanāñāṇaVijjā.
Sati is keeping conscious on the wholesome's object, to avoid unwholesome. The wholesome are 21 minds and 38 mind-factors. The wholesome's objects are 4 Satipatthana or 21 pabba.
Sampajanna is the perfect understanding in various perspective of the wholesome object.