pos-for-.net refers to Point of Service (POS) class libraries by Microsoft that allows applications developed in the .NET Framework to integrate with point of sale peripheral devices.
Using the UnifiedPOS (UPOS) industry standard, POS for .NET presents an open device architecture to allow .NET applications to interface with point of sale peripheral devices and conduct POS transactions. Additional information about POS for .NET can be found on Microsoft's POS for .NET overview web page.