Prosecutorial discretion is simply a measure of latitude in the degree to which the ICE may pursue a deportation case. Since no government agency has unlimited funds, prosecutorial discretion exists mostly to allow the agency to set priorities in the cases it submits to an immigration court.
Most of these priorities are low, which mean they can close “run of the mill” cases quickly and concentration on the more important ones. Now, as to which sorts of cases are more important, to a certain degree, that is a matter of opinion. The types of cases where the ICE uses prosecutorial discretion include public safety , border security, national security, and finally cases that are admitted on an ad hoc basis. It is the ones in the final category that discretion is used with least standard. Indeed, ask immigration officials about a hypothetical situation and ask them whether their cases should be closed through prosecutorial discretion, and you’ll find a considerable difference in opinion.
There are upsides to prosecutorial discretion. Naturally, not everyone who goes before the judge is allowed stay, but if your case is strong, they ruling might be in your favor. However “in your favor” should be elucidated. You may be granted a temporary stay, but you will not be granted the authorization to work, for example.
The downsides are that it’s a gamble. If you are not confident that your case is strong, it can backfire on you.
So we will admit that prosecutorial discretion can be arbitrary and finally unfair. And you can’t get it by asking for it. In fact that is something you should NOT do, because the chances are high that it will not be used in your favor. A better bet might be to simply fight your case the old fashioned way: in court through fighting removal or an appeal.