Fill with pattern and pattern overlay

What is the difference between pattern fill and pattern overlay. When shall one use either one?

Thank you in advance.

AC

They can both be used interchangably, depending on the circumstances.

But if you wanted your pattern to be partially masked with the layer below, then the pattern would have to be on it's on own layer, and then you could apply a layer mask.

Pattern overlay will cover and entire layer equally.

Rene