What is the difference between obsolete and irrelevant patches within the Patch Catalog?
Can such patches be removed?

 

Legacy ID:KA364929

OBSOLETE patches are the ones that have been replaced by newer patches from their respective Vendors.  Within the Bulletins SmartGroup, open a fairly outdated Bulletin, chances are to see the 'Obsolete' box checked with 'Superceded By' value listing the new patch(es) that should be considered instead.
Obsolete patches cannot be removed from the Catalog, as they are still valid, unless the Vendor has removed them from their site (see below).

IRRELEVANT patches are the ones that are left behind for the Filters which were removed from the existing Catalog.  When the filtering options are changed for an existing catalog, patches that no longer match the remaining filters are marked as 'irrelevant'. 
Also, it's possible for an Obsolete patch to become Irrelevant if the Vendor removes the particular patch from their site so it is no longer available to download.  In this case the depot object in the catalog for any patch in this state would be marked irrelevant during the catalog update job.

Irrelevant patches are not deleted automatically since they may be referenced in a patching job or by a BLPackage.

Irrelevant patches can be removed from the Catalog:
Right-click on the Catalog and select 'Remove Irrelevant Patches'.

Blcli commands also exist to remove irrelevant patches, but the name is misleading.
Here is Windows Catalog example (Note: the command actually removes irrelevant patches, not obsolete):

blcli_execute PatchCatalog deleteObsoletePatchesFromWindowsCatalog "/PathTo/WinCatalog"

Note that the blcli and the GUI methods only remove the patch objects from the TSSA database, they do not delete them from the file system of the patch storage location.