Interpretation of species' `constant` and `boundaryCondition` attributes.
`constant` value | `boundaryCondition` value | Can have assignment or rate rule? | Can be reactant or product? | Species' quantity can be changed by
--- | --- | --- | --- | ---
`true` | `true` | no | yes | (never changes)
`false` | `true` | yes | yes | rules and events
`true` | `false` | no | no | (never changes)
`false` | `false` | yes | yes | reactions or rules (but not both at the same time), and events