These are commands designed to help writers of classes or packages.
\AtBeginDvi & \AtEndDvi\AtEndOfClass & \AtEndOfPackage\CheckCommand\ClassError and \PackageError and others\CurrentOption\DeclareOption\DeclareRobustCommand\ExecuteOptions\IfFileExists & \InputIfFileExists\LoadClass & \LoadClassWithOptions\NeedsTeXFormat\OptionNotUsed\PassOptionsToClass & \PassOptionsToPackage\ProcessOptions\ProvidesClass & \ProvidesPackage\ProvidesFile\RequirePackage & \RequirePackageWithOptions