This document aims to describe the purpose, functionality, and implementation of a host for Polkadot's parachains. It is not for the implementer of a specific parachain but rather for the implementer of the Parachain Host, which provides security and advancement for constituent parachains. In practice, this is for the implementers of Polkadot.
There are a number of other documents describing the research in more detail. All referenced documents will be linked here and should be read alongside this document for the best understanding of the full picture. However, this is the only document which aims to describe key aspects of Polkadot's particular instantiation of much of that research down to low-level technical details and software architecture.