OPC is based on Microsoft's Component Object Model (COM). When you are using OPC connections across multiple machines, then DCOM (distributed COM) comes into play. Configuring DCOM is an issue which often causes a lot of grief and this section is designed to make this simpler.

Useful information on DCOM including an FAQ section can be from these web sites

I have also found another detailed explanation of DCOM configuration here.

Remote OPC DA - Quick Start Guide (DCOM).pdf
Using OPC via DCOM with XP SP2 v1.10.pdf