PIA: Primary Interop Assembly. A DLL distributed by the software manufacturer of a COM application having a programming interface that allows .NET to communicate ("interoperate") with the COM object libraries. In contrast to an IA (Interop Assembly) a PIA has been optimized to best reflect working with the native COM interface.
General information from the Microsoft documentation at https://msdn.microsoft.com/en-us/library/aa302338.aspx?f=255&MSPPError=-2147217396
What is a PIA?
Like any other managed assembly, an interop assembly is a collection of types that are deployed, versioned, and configured as a single unit. However, unlike other managed assemblies, an interop assembly contains type definitions (not implementation) of types that have already been defined in COM. These type definitions allow managed applications to bind to the COM types at compile time and provide information to the common language runtime about how the types should be marshaled at run time.
While any number of interop assemblies may exist that describe a given COM type, only one interop assembly is labeled the PIA. The PIA contains the official description of the types as defined by the publisher of those types. The PIA may contain certain customizations that make the types easier to use from managed code. The PIA is always signed by the publisher of the original COM type.
Any interop assembly that is not provided by the publisher of the COM types is considered unofficial and should be avoided. Because the types defined in such an assembly are not to be signed by the publisher of the PIA, they are incompatible with the definitions provided in the PIA.
Why are PIAs important?
PIAs are important because they provide unique type identity. The PIA distinguishes the official type definitions from counterfeit definitions provided by other interop assemblies. Having a single type identity ensures type compatibility between applications that share the types defined in the PIA. Because the PIA is signed by its publisher and labeled with the PrimaryInteropAssembly attribute, it can be differentiated from other interop assemblies that define the same types.
How do I make a PIA for my COM type library?
In most cases, creating a PIA is easy. The Tlbimp (Type Library Importer) tool provided in the Microsoft® .NET Framework SDK is capable of creating an interop assembly from an existing type library. The /primary switch on Tlbimp is used to create a PIA. All PIAs must be signed by their publisher so the publisher key must also be supplied with the /keyfile switch.
