Under SISSL, developers could modify and distribute source code and derived binaries freely.
Furthermore, developers could choose to keep their modifications private or make them public.
However, the SISSL is unique among OSI-approved licenses in requiring that "The Modifications which You create must comply with all requirements set out by the Standards body in effect one hundred twenty (120) days before You ship the Contributor Version."
If the Modifications do not comply, SISSL becomes a copyleft license, and source must be published "under the same terms as this license [SISSL] on a royalty-free basis within thirty (30) days."
Sun Grid Engine appears to still be covered by the SISSL.