Here are the security levels for Symbian OS 9 inclusing (S60 and UIQ)

1) Open Phone - can install both Symbian Signed and self-signed install files by default.
2) Open ‘Locked’ Phone, self-signed restricted - Symbian Signed by default only, self-signed via settings change. User intervention required to allow self -signed applications to be installed.
3) Closed ‘Locked’ Phone - Symbian Signed install files only.