Both Adjustment and Fill layers are considered special layers. However, they are not the same from a technical point of view.
Adjustment layers – They allow you to adjust (manipulate) the color, the exposure, or the lighting of the layers underneath them.
Fill layers – They allow you to fill layers with a color, a gradient, or a pattern.
I believe the confusion stems from the fact that a single button will bring up a menu listing both kinds of layers when you create these layers from the Layers panel. The first three top items are Fill layers; the others are Adjustment layers. This is less confusing when you use the Layer menu because Adobe clearly makes a distinction by listing separate commands for these two special layers.
Elements 7,10,11,12,13,14,15 - Photoshop CC 2017 - Lightroom CC 2015 Windows 7 Pro & Windows 10 Pro