is image retrieval by CNN the same classification or i need formula to do the similarity between query and our dataset?

hi,

Is image retrieval in CNN the same as classification or is there a special formula for image retrieval similar to query?

I have a program for retrieval of radiographs, I added the query image and the result was disease classification. If I want to retrieve images from dataset that is similar to the query, do I have to add an arithmetic equation or a specific code, or is the classification considered retrieval only for printing images?