The internal hotfix builds have passed our fully automated test suite - which includes thousands of functional, integration, unit and performance tests. The actual bug fixes have also passed strict manual quality assurance and performance tests. These builds have not passed our full manual regression testing cycle that we do before each official release. These releases will not include breaking API changes, database changes and high-regression potential changes or fixes. Upgrade from these builds to the consecutive official release is fully supported.
The bug fix builds can be installed by any customer having a license for the particular official version that precedes them - that's the same policy we have for the service packs. The downloads are activated only if you have first downloaded the major official version that precedes them.
List of release notes of internal hotfix builds available before Sitefinity 7.3 can be found in this forum thread.