Of the forty objects meditated upon as kammatthana, the first ten are ‘things that one can behold directly’, ‘kasina’, or ‘a whole’:
(1) earth, (2) water, (3) fire, (4) air, wind, (5) blue, (6) yellow, (7) red, (8) white, (9) enclosed space, (10) bright light.
The next ten are objects of repulsion (asubha):
(1) swollen corpse, (2) discolored, bluish, corpse, (3) festering corpse, (4) fissured corpse, (5) gnawed corpse, (6,7) dismembered, or hacked and scattered, corpse, (8) bleeding corpse, (9) worm-eaten corpse, (10) skeleton.
Ten are recollections (anussati):
First three recollections are of the virtues of the Three Jewels:
Next three are recollections of the virtues of:
(4) morality (Śīla)
(5) liberality (cāga)
(6) the wholesome attributes of Devas
(7) the body (kāya)
(8) death (see Upajjhatthana Sutta)
(9) the breath (prāna) or breathing (ānāpāna)
(10) peace (see Nibbana).
Four are stations of Brahma (Brahma-vihara):
(1) unconditional kindness and goodwill (mettā)
(2) compassion (karuna)
(3) sympathetic joy over another’s success (mudita)
(4) evenmindedness, equanimity (upekkha)
Four are formless states (four arūpajhānas):
(1) infinite space
(2) infinite consciousness
(3) infinite nothingness
(4) neither perception nor non-perception.
One is of perception of disgust of food (aharepatikulasanna).
The last is analysis of the four elements (catudhatuvavatthana): earth (pathavi), water (apo), fire (tejo), air (vayo).
Each kammatthana can be suggested, especially by a spiritual friend (kalyāṇa-mitta), to a certain individual student at some specific point, by assessing what would be best for that student’s temperament and the present state of his or her mind. source: wikipedia