A sub-release is a slightly supplemented or improved version of a software product offered for purchase rather than as a free update to the existing version.
A developer or vendor may choose to do a sub-release for a number of reasons. Sometimes a developer may experience feature creep to the extent that they will suffer financially if the update is provided without charge.
In monopoly situations, a company may decide that their market positioning makes it feasible for them to charge for the product. A developer may simply compare the updates features to features of the competition and decide that a sub-release is justifiable and that the market will support it.
Sub-releases have been offered by many companies at one time or another. A well-known example is Microsoft’s Windows 3.1, an updated version of Windows 3.0 that added support for TrueType fonts and peer-to-peer networking.
According to some experts, sub-releases paved the way for paid update subscription models.