Your Pickaxe will use Perplexity whenever it decides it is required. That being said, you can write your prompt in such a way that it ALWAYS uses Perplexity.
The Knowledge Base will be used whenever there is information that is ‘deemed relevant’. That means, whenever information from the knowledge base exceeds the relevance score threshold (a setting within the Learn tab), it will be referenced.
In the builder, you can click the magnifying glass icon under the responses to see exactly what parts of the Knowledge Base were used.