Where does discretion become a failure to act?

I know i've found a FL statute on this before but darned if i can find it now. I'm not a cop. What i'm wondering is when does a law enforcement officer exercising his discretion rise to a "failure to act" or whatever the appropriate phrase is? I'm talking about criminal offenses. Do you have discretion on whether to arrest for a misdemeanor? what about a felony? Where is the line officially/legally drawn? Can anyone clue me as to which statute outlines when an officer knowing fails to act rises to crime?

And no this isn't case specific, there's no one i've got it in for whose failed to act. :)

What got me thinking about it again was the recent comments in the "LCSO Deputy does the walk of shame thread". Specifically the ranting response about anyone in the brotherhood who would arrest another for DUI is a rotten rat, period.

BTW i know opinions run strong on both sides about whether to arrest another officer for DUI or not. I'm not trying to open that can of worms. What I'm interested in is when does a Florida law enforcement officer exercising his discretion cross the line and becomes a failure to act. Whether it be DUI or any other offense.

If the statute says, "...will affect an arrest" versus "...may affect an arrest".

You do not have a requirement to arrest on most charges. As an example, many officers will seize marijuana or small amounts of cocaine and impound it into evidence in lieu of affecting an arrest on an otherwise law abidng citizen.

DUI arrests are aften diverted if their is a sober driver available. In other words, if the driver can call his boss, spouse, girlfriend, or buddy to come pick him up in a reasonable amount of time, there is no need to make an arrest.

Domestic Violence and warrants are generally the only cases where you really don't have any discretion. Common sense has to dictate, though.