@Baldimo,
It's attached to several:
1. The US Constitution which is the supreme law of the land. The 14th amendment.
2. The 1990 immigration act which gives preference to persons who have family members working in the US.
3.HR 3547 which set funding for the detentions and deportation but only included enough money to deport about 400,000 per year. It's estimated there are over 11 million undocumented aliens in the US.
4. HJ Res 124 which set Border Patrol at current levels.
Since Congress has not provided enough money to deport all illegal aliens then it is up the the President to decide which to deport. It is hardly out of left field to decide that certain people are not a priority when it comes to deportation. Would you rather we not focus on using the limited funds to deport criminals?