Jetsün Chokyi Gyaltsen:
The definition for a close placement by mindfulness is:
An exalted knower on the path, that is contained in either mindfulness or wisdom, and which meditates by investigating the general and specific characteristics of each of the four: Body, feelings, main consciousness and phenomena.
Vasubandhu in his Treasury of Knowledge (Abhisamayaalamkara):
“The close placement by mindfulness is wisdom.”
Examples for general characteristics: Impermanence, misery, empty and selfless.
Everything compounded is impermanent
Everything contaminated is misery
All phenomena are empty and selfless
Purpose of meditating on the four close placements:
To reject what has been identified by the four noble truths as that to be abandoned, and to integrate what has been identified by the four noble truth as that to be accepted.
The purpose of meditating on the four close placements (of wisdom) by mindfulness is to set us on the path by clearly identifying what is contained in the first two noble truths, and rejecting this, and by identifying what is contained in the second two noble truths and aspiring towards them.
Realizing the four noble truths in relation to ourselves generates renunciation, and realizing them in relation to others generates compassion.