The concept of brand suitability is related to brand safety. Brand safety is about avoiding inappropriate content, and all audiences generally agree upon what falls into those categories. Brand suitability, on the other hand, is unique to an individual brand. Brand suitability is about matching your ads to content that aligns with your particular values, identity, tone, and other subjective qualities.
For example, a vegan brand may not consider an app about hunting and fishing a suitable placement for their ad, even though hunting and fishing aren't inherently objectionable topics to everyone. A website that is very casual and humorous in tone may not be brand-suitable for a company whose brand voice is more serious. Content about how to make a cocktail may not be brand-suitable for a company who makes educational toys for children.
Brand safety is about protecting your brand from harm. Brand suitability is about supporting and developing your specific brand identity by understanding which types of content resonate with your brand, while avoiding associations with content that doesn’t fit with your brand.