The law has recently undergone a major change with regard to Petitions to Seal and Destroy Arrest (or Detention) Records, particularly if the defendant has not been convicted.
The Old Way (Penal Code 851.8):
In the old days (meaning prior to January 1, 2018), in order to seal and destroy a record of arrest, the petitioner was required to prove "Factual Innocence" of the crime for which he or she was arrested. Penal Code 851.8(b) defines factual innocence as "no reasonable cause exists to believe that the arrestee committed the offense for which the arrest was made."
In other words, a Petitioner would have to prove to a judge that the arresting officer was acting unreasonably when he arrested her! This was an extremely high burden for a Petitioner to Meet, and rarely successful.
The New Way (Penal Code 851.91)
Thanks to California Senate Bill 393, as of January 1, 2018, Petitioners do not have to prove factual innocence to seal and destroy their arrest records! Instead, an arrestee may have his record sealed and destroyed as a matter of right if the following are true (with a few narrow exceptions):
1. Petitioner was not convicted (Penal Code 851.91); or
2. Petitioner was enrolled in and successfully completed a court diversion program (Penal Code 851.90).
Not everyone knows about this law yet! Get in early, before the form is changed, and you may even get a finding of factual innocence.