Some Packages are Add-On Packages. These Packages can only be used to extend existing Base Packages; as a result they cannot be used to create new Libraries.
For example, consider the following two Packages:
- stsim: This Base Package is for developing state-and-transition simulation models using the ST-Sim model.
- stsim-stockflow: This Add-On Package provides optional additional functionality for the base ST-Sim model.
You can use the stsim Base Package to create new ST-Sim Libraries in order to run the ST-Sim model. But stsim-stockflow cannot be used to create new Libraries and cannot be run on its own. To use the stsim-stockflow Add-On Package, you must first install it, and then enable for each Library that you want to extend.
To use an Add-On for a particular type of model you must first install the Add-On Package using the Package Manager. Once installed, that Add-On can then be enabled for any Library created for the model it extends.
Continuing the example above, if you install the stsim-stockflow Add-On Package then it will be available as an Add-On for all of your stsim Libraries (and only your stsim Libraries.)
Once an Add-On Package is installed you can use the Package Manager to see which Base Package it extends. The image above shows the Package Manager with three Packages installed. As you can see, both dgsim and stsim are Base Packages while stsim-stockflow is an Add-on to stsim.
Add-Ons are enabled on a per-Library basis. To enable an Add-On for a Library, select the Library Properties item from the File menu, then select the Add-Ons tab.
The image above shows that the stsim-stockflow Add-On is enabled for the current Library. The Add-On Package's Name, Description, Installed Package Version and Minimum Package Version are also shown. See the Add-Ons System Property for explanation of these Package attributes.
The Add-On tab on the Library Properties screen only appears once you have installed at least one Add-On Package suitable for your Library.
When you enable an Add-On, the Properties for that Library, and each of its Projects and Scenarios will optionally be extended. The image above shows the Scenario Properties after the stsim-stockflow Package has been enabled. As you can see, the here are new "Stocks and Flows" Properties available in the Advanced tab.