Using a submodel to specify a 'satellite' relationship

When you pass information out of fixed-membership multiple-instance submodel, it appears as an array with a fixed number of elements. You can extract the value for any one element using the element([array],index) function, and the element you extract will correspond to the instance in the fixed-membership submodel. This makes it possible to select values for one variable on the basis of the value for some other variable.

However, when you pass information out of a population submodel or an association submodel, all you get is a list of values: each value is not tagged with the index (instance number) of the submodel instance that produced it. Thus, if you had a population submodel for a population of rabbits, and you wanted to find the total weight of all rabbits over two years in age, you couldn't do it simply by looking at a list or lists coming out of the submodel.

The satellite submodel is a way of making this possible. It is a submodel with a single role arrow pointing at it (in contrast with an association submodel, which has two role arrows). At most there will be one instance of this submodel for every instance of the parent submodel (the one the role arrow comes from). But, as with a conditional or an association submodel, you can place a condition model element (the question mark symbol) inside it, and this can limit the number of instances to be some subset of the maximum number possible. The condition you insert is then specified to restrict the subset to that which you require.