It is to let the mind continue with its objective reference. The reason for calling the particular content of intense concentration an idea is that in practicing concentration, one holds the mind to what is postulated by intellect, and this is an idea.What is intense concentration? It is one-pointedness of mind which continues with an idea, and its function is to become the basis for awareness [shes pa].-Abhidharmasamuccaya
Moreover, although there are countless things which may appear as that on which one can concentrate, in brief, there are four:
1. The objective reference which purifies the conduct of lifeThe objective reference with which the wise deal is said to be limitless and refers to such topics as the five constitutive elements, the three levels of existence, the six perceptual spheres, dependent origination and so on.
2. The objective reference which purifies emotions
3. The objective reference which are the (four) encompassing activites (of loving kindness, compassion, joy and equanimity)
4. The objective reference with which the wise deal
Nowadays, some people who take the Buddha's words out of context and make a display of meditation, meditate by concentrating on what appears before their visual perception, but Asanga has stated very clearly that concentration does not take place in sensory perception but in categorical perception, and the objective reference is not the visible object that is present in sensory perception, but its precept.
Now if you think that the object of intense concentration must necessarily be a real object, this is not the case. You can concentrate on anything -- be it a real or fictitious object. If you work on it intuitively, you get from it a pure and non-conceptualized vision.
The specific function of intense concentration is said to provide a basis for an awareness in which one has a state of mind settled in itself, ( a mind) taking every individual aspect of the perceptual situation as it is and never occurring in a vacuum, even though attention may shift within this perception (from one aspect to another).Whether it is real or not,
Whatever becomes truly familiar
When you have become fully conversant with it
Results in a clear feeling of presence without subject-obejct dichotomy.-Pramanvarttika
Such statemtents are made over and over again. Therefore, those who want to have liberation must take these works as the Sublime Instruction.From having known that the harges of emotions are overcome
Through a widened perspective which is together with calm,
One must first become calm, and this comes
When one is not attached to the worldDepending on conduct there is intense concentration, and depending on intense concentration, there is appreciative discrimination.-Bodhicaryavatara-Vinayasastra