"Although elaborative inferences are not required for textual coherence, they serve to enrich the mental representation of the text and therefore make it more memorable." (Elaborative (or extending) inferences. Retrieved from nationalstrategies.standards.dcsf.gov.uk)
While elaborative inferences aid in, and expand comprehending, they are not required for comprehending. For example, when reading the following: "She saw smoke pouring out of the neighbor's house, and immediately dialed 911." You may infer that she called on a cell phone. This inference would be elaborative. This inference is not required to comprehend the sentence, but it does expand a reader's comprehension by providing a more detailed mental image.