The other remedies
Sealing, reduction, and automatic relief are different things
Sealing is the remedy that actually restricts access, and it applies to arrests, not convictions. Under Penal Code 851.91 a person arrested but never convicted, because no charges were filed, the case was dismissed, or there was an acquittal, may petition to have the arrest record sealed as a matter of right in most cases. That is a stronger remedy than 1203.4, and it is available to a different group of people.
Reduction is often worth more than dismissal. Penal Code 17(b) lets a court reduce a “wobbler” felony to a misdemeanor, which changes what the conviction is rather than adding a note to it, and can restore firearm rights that a felony took away. The usual sequence is to reduce under 17(b) first and then dismiss under 1203.4.
Automatic relief now exists and is the reason many people do not need to file anything. Penal Code 1203.425 directs the Department of Justice to identify eligible arrests and convictions and grant relief without a petition, on a schedule set by the statute, with significant exclusions including serious and violent felonies and offenses requiring sex offender registration. Automatic relief is not a substitute for checking your own record: it does not reach every case, and it has the same limits as any other relief on disclosure, firearms, and immigration.
A certificate of rehabilitation under Penal Code 4852.01 and a Governor’s pardon are separate forms of relief with their own eligibility rules and effects. Check eligibility for dismissal under 1203.41 even where the sentence was served in state prison; a certificate or pardon is not the only possible route simply because prison was involved. Neither a certificate nor a pardon erases the conviction.
Sources: Penal Code 851.91 — sealing an arrest that did not result in conviction; Penal Code 1203.425 — automatic conviction record relief; Penal Code 17(b) — reducing a wobbler felony to a misdemeanor; Penal Code 4852.01 — certificate of rehabilitation.