Which track output should I use first?

Use output A first.

In the standard CSB1 configuration, output A is used for DCC MAIN operation and output B is used as the PROG, or programming track, output.

That means:

• Use A for normal train running.
• Use B for programming track work such as reading and writing decoder CVs.

Imported from: DCC-EX CSB1 and Engine Driver FAQs