SysML has a lot of rules (and they all exist to serve the very useful purpose of giving your design unambiguous meaning from one reader to the next). However, you sometimes need to express information on a diagram in an unconstrained way as a block of text. You can do this with a comment.
A comment is, in fact, a model element. It consists of a single attribute: a string of text called the body. You can convey any information you need to in the body of a comment, and you can optionally attach a comment to other elements on a diagram to provide additional information about them. You can use comments on any of the nine kinds of SysML diagrams.
The notation for a comment is commonly referred to as a note symbol: a rectangle whose upper-right corner is bent. You use a dashed line to attach a comment to other elements (as shown at the bottom of the BDD in Figure 3.27). If you need to, you can attach a comment to several model elements simultaneously by using a separate dashed line for each one.
Figure 3.27 Comments on a BDD
Modelers sometimes put freestanding comments with hyperlinks on a diagram to enable readers to quickly navigate to a related diagram in the model (or to an external document). An example of this is shown in the upper-left corner of the BDD in Figure 3.27. To be clear, though, this capability is a function of the modeling tool you use; not all tools do this. And SysML itself says nothing about this capability.
SysML defines some specialized kinds of comments: rationale, problem, and diagram description. These appear as a note symbol with the respective stereotype preceding the body of the comment. Figure 3.27 shows an example of a diagram description comment in the upper-right corner of the BDD. Modelers often use rationale comments in conjunction with requirements relationships and allocations. I discuss these topics in detail in Chapters 11 and 12.