While most people starting out with Photoshop tend to utilize Opacity slider and leave Flow untouched, knowing the difference between the two will allow you to select the one that will perform best for a given brush application.
One of the most common questions from Photoshop users, and not just beginners, is "What is the difference between the Opacity and Fill options in the Layers panel? This is a two part tutorial: The first "The Difference Between Opacity And Flow" using the brush tool in photoshop. The second "The Difference Between Opacity And Fill" in the layers panel in photoshop.
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