A regression bug is an issue that was introduced in a Maintenance Release but did not exist in previous releases of that code stream.
While Fix Packs provide important fixes and IBM strongly recommends applying the latest Fix Pack available for a Maintenance Release, IBM still recommends that customers upgrade to the latest Release + Fix Pack combination to receive the broadest set of fixes available. We encourage customers to evaluate Notes / Domino 9.0 Social Edition and the upcoming 9.0.1 release scheduled for Q4. By providing a small number of fixes, customers are able to accept fewer code changes with lower risk, allowing them to "patch" an older Maintenance Release until a more extensive upgrade to the current Release is possible.
They go through the same level of fix, regression and interoperability testing that occurs with Maintenance Releases. Fix Packs are released periodically between Maintenance Releases to provide a greater level of stability for customer environments. IBM strongly recommends that customers running Notes/Domino 8.5.3 upgrade to this latest Fix Pack since it addresses a small percentage of defects that impact the broadest set of customers. Notes/Domino 8.5.3 Fix Pack 5 is a scheduled Fix Pack of low-risk, high-impact fixes to help customers safely avoid known issues.
Lotus Notes/Domino 8.5.3 Fix Pack 5 Release Notice Notes/Domino Fix List - Lotus Notes/Domino 8.5.3 Fix Pack 5 Release Notice