Ryan Holiday defines a commonplace book thusly:

A commonplace book is a central resource or depository for ideas, quotes, anecdotes, observations and information you come across during your life and didactic pursuits.

More or less, my commonplace book exists within both Ulysses and Day One. With the former holding my ideas and quotes, and the latter holding my observations and information.

Over on The Sweet Setup, my friend Chris Bowler wrote a three-part series on keeping a commonplace book. Part One gets in to the what and why, and parts two and three get in to the details of how to toss all that stuff in to Day One.

Keeping a Commonplace Book with Ulysses or Day One