SysML is a offshoot of UML 2.0 that aims to achieve Model-Based Systems Engineering (MBSE ). It supports specification analysis, design, verification and validation of a broad range of systems including hardware, software, information, processes, personnel, and facilities.
SysML is codified into ISO/IEC 19514:2017 - "Information Technology - Object Management group systems modeling language" by the Object Management Group and ISO.
SysML 2.0 is coming out SOON(tm) and will represent a serious, massive improvement over the current standard (my opinion). It will include a further break from UML, but in doing so a more consistent and refined language. And, most importantly, a textual language and standard API to go along with the Graphical language.
SysML uses SysML Blocks. Is a strengthened "Class" from UML. See also SysML Block Property Types andStandard SysML Block Extensions
BDD - How parts are related to one another
IBD - how parts within a single Block are connected.
SysML has many ways to illustrate behavior.
SD - shows how messages are passed in order between participants.
SD - also from UML. Describes states blocks can have, and how they transition between them.
AD - for unconstrained behavior, use cases, and functional requirements.
REQ - What the requirements are, how they are related, what parts of the system they relate to, and how they are verified.
PAR - Shows how blocks and properties are related to one another through the use of mathematical constants
There is also the concept of allocation between the parts of diagrams. Parts of the activity diagram can become associated with parts inside an internal block diagram.
Requirements can be related ("satisfied") by parts in ibd or act diagrams.