Starting with the v2018.3 release, the Core Activities Pack is split into two distinct packages, UiPath.System.Activities and UiPath.UIAutomation.Activities. This has been done with the intention of offering you more control over which activity versions you use in the execution of your automation projects. In correlation with the Project Dependencies feature, this split enables you to choose exactly the version of activities you want to use as a dependency for you project.
Whenever a project created with a version of Studio older than v2018.3 is opened with Studio v2018.3 or higher, the project is automatically migrated and the new System and UIAutomation packages replace the old Core pack.
Once the migration is performed, you cannot revert the project to use the old Core pack.
Starting with Studio v2018.3 or higher, you cannot install the Core package from the Package Manager. However, rest assured that the UiPath.System.Activities and UiPath.UIAutomation.Activities packages contain all the activities that were in the old one. Additionally, from v2018.3 on, you can use any combination of activity packages regardless of what version of Studio you are using at that moment.
Once a project is published with v2018.3 or higher, it can no longer be executed by using a v2018.2 or lower Robot.
For more information on how this change impacts Studio, you can read more here.