When is it appropriate to use adhesive instead of thin-set mortar?

Using adhesive instead of thin-set mortar is appropriate for specific tile types, including certain vinyl or rubber tiles. These materials often have unique installation requirements that necessitate the use of a specially formulated adhesive to ensure proper bonding and performance. Adhesives designed for these tile types may offer flexibility, moisture resistance, and other characteristics that thin-set mortars do not provide, making them essential for achieving a successful installation.

In contrast, large format tiles require specific considerations but do not exclusively require adhesive over thin-set mortar. While thin-set mortar can also be suitable for large format tiles when applied correctly, specific adhesives are mandatory for particular tile materials that might include vinyl or rubber and could compromise their integrity if thin-set is used. Therefore, the presence of specific requirements associated with certain materials highlights why the correct answer pertains to this specialized use of adhesives.