Impression materials used for indirect restorations: 

It is crucial to have accurate impressions that reproduce the characteristics of hard and soft tissues.
In general, the final quality of the impression depends on :
 the preparation features,
especially regarding the location of the margins 🔍 in relation to the gingival sulcus, the quality of tissue retraction (when necessary), the sharpness of the margins and the degree of surface smoothness;
 the quality of the impression material,
since the level of accuracy of the impression varies significantly between materials;
 and finally a thoughtful impression-taking technique,
since using correctly the materials is just as important as the material’s quality.
This is a brief introduction of nonaqueous elastomers :
In our dental school clinical activities, addition silicone is the most used material due to its outstanding detail reproduction, excellent dimensional stability and compatibility with the recommended impression techniques. Although the cost of this material is high, especially when compared to other options.
Certainly, it is possible to obtain acceptable impressions with other elastomers, however, the chance of failure is greatly increased or the impression technique requires an increased number of steps. Polysulfides, e.g., available in a single (light) consistency require the fabrication of custom trays (impression copings), spending more of the clinician’s time.
With addition silicones, such a maneuver is unnecessary, since it is possible to combine materials with different viscosities (heavy, light) in the same impression. In this case, high-viscosity material act as a custom tray, adjusting the characteristics of the prefabricated tray to the particularities of the area to be reproduced, while the low viscosity, more fluid material responsible for copying the details of the dentogingival structures.

Note in the table below 👇🏻, a comparison of the most relevant properties of each material.
You can check the post of impression techniques 👀