"What is a Maintenance Level Update (MLU)? A Maintenance Level Update (MLU) is a new distribution kit format that contains cumulative fixes and enhancements to the product since the Pega 7.1.0 release. It is not a full product release. You must have an earlier version of Pega 7.1.x installed (for example, if you are using Pega 7.1.3, you can apply the MLU for Pega 7.1.6). Cumulative release notes are posted on the PDN at https://pdn.pega.com/release-notes. "
For the full Pega 7 Platform, an update is a new distribution kit that contains cumulative fixes and enhancements to the product since Pega 7.1.0; it is not a full product release. In contrast, upgrades are full product releases. If you need to move from any version prior to Pega 7.1.0, you must upgrade. To move from any Pega 7.1.x version to the most current release, run the update. See the Upgrade Guide or the Update Deployment Guide for more information about the differences between upgrades and updates.