Microsoft Knowledge Base Article 194513 contains the following summary:
This article discusses the structure and functionality of a boot file within the Domain Name System (DNS) schema. The first file in a DNS server is the boot file. However, the boot file is not defined in the Request for Comments (RFCs) and is not needed for a DNS server to be RFC compliant. Boot files are actually a part of the Berkeley Internet Name Domain (BIND) implementation of DNS. Boot files determine the startup behavior for DNS servers that incorporate the BIND specifications. A Microsoft Windows 2000 DNS server is compatible with the BIND implementation that supports the use of boot files.