Which licensing type allows for the combination of open-source and proprietary code?

The Mozilla Public License (MPL) is designed specifically to allow the combination of open-source code with proprietary code. This is achieved through a unique copyleft requirement that is more permissive than traditional licenses, such as the General Public License (GPL). The MPL allows developers to create derivative works that can be licensed under different terms, thus enabling them to integrate proprietary components into their applications while still maintaining the open-source nature of the original code.

Under the MPL, files that are modified and distributed must remain open-source, but the entire project does not need to be open, allowing developers the flexibility to keep some parts of their work proprietary. This contrasts with licenses like the GPL, which typically require that any derivative works also be released as open-source, thus discouraging the integration of proprietary components. The Lesser General Public License (LGPL) has a similar focus on allowing some proprietary use but does so under different terms.

The Apache License also permits the combination of open-source and proprietary code, but the licensing approach is different than that of the MPL. While both the MPL and Apache License encourage flexibility for integrating various code types, the structure and obligations for modifications and distributions are what distinguish the MPL's specific capacity to blend these code types efficiently.