Slab Serif group is quite often known as Serif's sub-category, without separating into two specific groups. Indeed, their incredible geometric forms have strong rectangular, very similar to blocks serifs that are often the same width with the base line. They are connected to one another at a right angle.
Nonetheless, Slabs have an important difference. The lighter or in some cases nonexistent contrast, so typical for these fonts, provides the greatest readability if it is needed to print the large amount of text, especially on low-quality paper.
For added convenience AZfonts has decided to separate Serifs and Slab Serifs.