A product requirements document (PRD) is a detailed outline of all functionalities a software product must fulfill when being delivered.
The PRD describes the magnitude of a product and serves as a record of all features that must be realized or in retrospective must be found in the product.
Each functionality is characterized as an individual use case and needs to explain its purpose to establish an accurate understanding for a cross-functional team to take measures during the implementation process.
Additional conditions such environment requirements, e.g. hard- and software utilization of the end user, constraints and dependencies also need to be specified in the PRD.