OPC Data Access (OPC DA) is an OPC Classic specification for exchanging current industrial process values between a server and client, including timestamps and quality information. It uses Microsoft COM/DCOM technology and is often found in older control and data-collection systems. OPC UA is a newer, platform-independent architecture with a broader information model and security framework. A site moving from OPC DA to OPC UA should check the actual tags, data meaning, access rights and gateway behaviour.
Related: OPC UA · OPC Foundation: OPC Classic.