This paper describes new capabilities in the Systems Modeling Language that reduce the complexity of specifying systems through componentization, and increase the range of systems that can be specified. Modelers can identify portions of components available for connection to other components, and specify how systems make use of them. This reduces the complexity of specifying system by lowering the number of ways components can be connected, partly by hiding portions of components, and partly by limiting how exposed portions can be connected to others. The paper introduces basic SysML concepts and notations for specifying components and assembling them into a system. It covers capabilities added to SysML enabling a wider range of systems to be modeled, through detailed specification of the portions of components available for connection to other components, and detailed specification of the connections between them. These capabilities are described by relating models to potential systems built to meet the specifications, an approach typical in specifying semantics of formal languages. The paper is intended for those concerned with increased precision in SysML concepts, such as builders of model analyzers, checkers, and other automated support for systems engineering. It also enables more reliable interpretation of the SysML models generally, for example, during manufacturing and other stages of the product lifecycle. Examples are given in SysML notation for each concept.