info.txt

Eagle Layers: https://www.autodesk.com/products/eagle/blog/every-layer-explained-autodesk-eagle/

Layer 20: Dimension
The Dimension layer has several purposes, the first of which is to specify the outline of your board.
Secondarily, you can also use this layer in your design rules to keep copper pours away from the edge of your PCB.
Some fab houses like OSH Park will use this Dimension layer to generate a board outline Gerber.
This will serve as the exact shape of your PCB when they cut it into a fabrication panel.

Layer 46: Milling
This layer is solely dedicated to the milling of holes, inner cutouts, and any other kind of contour that needs to be cut by your manufacturer.
Keep in mind, this layer is not meant to define the dimensions of your board; you’ll need to keep that data on Layer 20. 
And unlike the Dimension layer, which can interact with your design rules, the Milling layer does not.

Layer 29-30: tStop/bStop
These two layers indicate where your solder mask should not be applied.
When placing either through-hole or surface mount components, these parts will typically include a solder mask expansion area that resides on these two layers.
Defining an area that you don’t want soldermask applied to will provide space on your copper for soldering parts.
You can also use this layer to draw custom structures, like heatsinks or gold artwork by exposing specific areas of copper.

Layer 41-43: tRestrict/bRestrict/vRestrict
tRestrict and bRestrict are used to indicate where traces or copper should be removed on your board layout.
By default, when you fill in a polygon with copper over a restrict layer, the restricted section will remove any copper.
This layer also comes in handy when using the autorouter in EAGLE, as it can prevent traces from being routed in defined areas.
vRestrict will indicate where vias should not be placed, and this layer will also prevent the autorouter from placing vias in defined sections.