It's often useful to extract a map from an image or from another source map. All maps can be extracted, except the {PBR?Base Color|Diffuse} map which is usually a source to extract other maps from.
In some cases, extraction is quite exact and similar to a computation, like when extracting Ambient Occlusion from a Displacement map. In other cases it's more of an approximation, providing a starting point upon which further tuning is required.
Let's start by selecting the Editor you'd like to extract into, from the map tabs above the Editor.
To extract a map you can use the Extract tool which is available at the top of the Map Tools available for the current map.
Most maps can be extracted from different types of sources, so the first step is to select the Method of extraction. Other maps, like Metallic, can only extract from a fixed map, usually Base Color. If this the case, there won't be a Method setting, just continue to the next step.
Now select the actual source image by clicking in the Source drop menu. The available options depend on what was selected in the Method setting and include:
After selecting the source image, extraction will automatically start.
The extracted image will be of the same Width and Height as the source image. If by chance a map is currently open in the destination Editor and is of different size, a window will appear asking which size to keep.
After extraction starts, new settings may appear right below the Source setting, which allow you to tune the extracted map. These settings depend on which map you're extracting: please check the tool information by leaving the mouse over the Extract tool header and clicking the ( i ) button that appears.
The extracted map may need to be further refined or retouched with other available tools, like the Keying or Paint tools.
You may want to see the following video tutorial about automatic map extraction: