Optional vs. Required Flows

According to the SysML Specification version 1.6 (beta), the «optional» stereotype may be applied to parameters on an activity diagram indicating that the parameter is “not required to have a value for the activity…to begin or end execution.”  In this case, «optional» may be displayed next to the parameter on the diagram.  Use of an «optional» stereotype on a parameter implies that the lower multiplicity is zero.  If the lower multiplicity is one or greater, then the parameter is considered required.

Reference:

Object Management Group (2018) OMG Systems Modeling Language Version 1.6. Needham, MA: OMG. Para 11.3.2.6.