To simulate continuous tones (in images) screening is used. Screening can be made in different ways.
AM, amplitude modulated screening uses dots that are placed within the same distance from each other but are different in size, i.e. the frequency is constant whilst the amplitude is variable. FM screening, frequency modulated screening on the other hand uses dots of the same size and the distance between the dots is varied, i.e. the frequency is variable and the amplitude constant.
The positioning of the dots is said to be made randomly, or more correct: they are placed in a random way but calculated carefully so that the dots are placed as optimally as possible not causing clusters of dots etc hence to avoid print unevenness (mottle) in homogenous tone areas.
Hybrid screening, cross-modulated screening is the combination of AM and FM screening where an algorithm defines the optimal use of each screening technique according to the content of the image or oriented to different parts of the tone range.