We need to extrude part of our model 2mm away from another face. In this case we don't have a sketch plane we can use. Rather than creating a plane, let's look at the hidden gem!
Here is another look at our dilemma.
Let's review our Extrude dialog box. If you look at the top you see that we have a pull-down menu for "From".
I also took a snap shot of it expanded.
You can use this "From" section in the Extrude dialog box to extrude from your Sketch plane (this is the default), a Surface, Face, or Plane, any Vertex in your part or a Offset. In my example we need to offset the Extrude 2mm so here it is in action.
I use the "From" option all the time to save me from have to creat extra reference planes in my parts.