the characteristic of two (or more) licenses according to which the codes distributed under these licenses may be put together in order to create a bigger distributable software. [emphasis added]
License compatibility: The characteristic of a license according to which the code distributed under this license may be integrated into a bigger software that will be distributed under another license. [emphasis added]
With individually licensed source code files, multiple non-reciprocal licenses (such as permissive licenses or own proprietary code) can be separated, while the combined compiled program could be re-licensed (but that is not required). Such source-code file separation is too weak for copyleft/reciprocal licenses (such as the GPL), as they then require the complete work to be re-licensed under the reciprocal license as being derivative.
You may Distribute your Modified Version as Source (either gratis or for a Distributor Fee, and with or without a Compiled form of the Modified Version) [...] provided that you do at least ONE of the following: […]
(c) allow anyone who receives a copy of the Modified Version to make the Source form of the Modified Version available to others under