Explanation
There are two types of upgrades available for CCE: Common Ground and Standard. A Common Ground upgrade is a type of upgrade that allows you to upgrade from one major release to another major release without having to rebuild your configuration. A Common Ground upgrade preserves your existing configuration and data, and applies the new features and enhancements of the new release. A Common Ground upgrade is supported for certain releases and deployment models, and requires a specific upgrade path. A Standard upgrade is a type of upgrade that allows you to upgrade from one minor release to another minor release within the same major release. A Standard upgrade also preserves your existing configuration and data, and applies the bug fixes and minor enhancements of the new release. A Standard upgrade is supported for all releases and deployment models, and does not require a specific upgrade path12. References:
Cisco Unified Contact Center Enterprise Installation and Upgrade Guide, Release 12.6 (1) - Upgrade Overview2 Cisco Unified Contact Center Enterprise Installation and Upgrade Guide, Release 12.5 (1) and 12.5 (2) - Upgrade Overview3

Two types of upgrades available for CCE are Technology Refresh and Standard. Technology Refresh upgrades replace outdated hardware and software components with the latest versions, while Standard upgrades include features and bug fixes.