Some PHIDType classes have missing application or icon specifications. Today, this primarily means that feed stories don't show icons.
After D14323, you can find PHID types with missing applications in Config → PHID Types. PHID types which are clearly part of an application but don't show anything in the "Application" column have missing methods.
D14324 is an example of how to add a method call and test that it is correct.
These patches are straightforward. We will review and accept work here from contributors if you're looking for something easy to get started with.
By the end of this, we'll probably hit 100% coverage and then make the base method abstract to force implementation and prevent this from happening again in the future.