Top and Bottom are nothing more than the beginning and closing layers of the 3D entity.
Top and bottom thickness are what it sounds like and this setting adjusts how many mm of 100% infill layers Cura will use to create them.

Let’s do a simple exercise. Import a cube into Cura3D, set the parameters as the following picture, and go to the 10th layer.
Set 0.1mm layer height and 1mm Top/Bottom thickness. You’ll notice that the 10th layer has a 100% infill rate, rather than the 11th, which is the Infill.

10th layer. 100% infill density.
11th layer, 25% infill density

There is no specific rule on how to set the top and bottom thickness because it’s based on what are the needs of the 3D entity.

Printer settings cannot be “one-size-fits-all” since 3D printers vary from one type to another.

The top layer affects the resistance of empty objects. It is recommended to set a solid top thickness when the infill density is low (like 6%). Another advice is to increase the shell thickness with these kinds of empty objects.

The top layer is irrelevant when you set a 100% infill density.

There are cases in which the infill density influences the aesthetic finishing of the printer object, like for translucent or transparent objects.
One needs to know which parameter to activate or deactivate, in order to prevent unpleasantly stains of the material on the object.

Lampshade, and similar objects, are made with high top/bottom thickness, high shell thickness, and almost none infill density. This creates a perfect translucent object.

A useful trick when setting the top/bottom layer is to use multiples. To create 6 top layers, I set 0.15mm layer height and 0.9mm top layer thickness.

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