# Computing initial values for pools of instances

In SBML Level 3 Version 1,

s are used to assign initial values to variables of a model, that are **InitialAssignment**

s, **Compartment**

, **Species**

, or global **SpeciesReference**

s. In order to assign the value of a specific type of species, **Parameter**

s in multi Version 1 also contain an element **InitialAssignment**

. The assignment sets the initial quantity of instances that fulfill a certain selection using the mathematical expression provided. The value provided by the **SpeciesTypeInstanceChange**

overides the value provided by the attributes **InitialAssignment**

or ** initialAmount **

** ***initialConcentration*

of the relevant **SpeciesTypeInstance**

.

## InitialAssignment

In order to assign the initial values to entity subpools, defined by specific state and connectivity, the element

of SBML Level 3 Version 1 core is linked to a **InitialAssignment**

.
**SpeciesTypeInstanceChange**

## SpeciesTypeInstanceChange

As all elements derived from

, a **SBase**

can link to **SpeciesTypeInstanceChange**

and **Notes**

, and carry a **Annotation**

, and an ** metaid **

** ***sboTerm*

. It targets a **SpeciesTypeInstance**

through its ** ***speciesTypeInstance*

attribute. The change is computed using a MathML construct, as is always the case in SBML Level 3 Version 1.
## Complete example of an initial assignment

The following example presents the assignment of the initial value for the subpool of

defined as *species1*

. This value is computed as the product of the parameters *speciesTypeInstance1*

and *x*

, defined elsewhere.
*y*

Note the empty

, used when the software is unable to use the package multi Version 1.
**Math**

<species id="species1" boundaryCondition="false" hasOnlySubstanceUnit="false" constant="false" compartment="compartment1" initialAmount="1000" xmlns:multi="http://www.sbml.org/sbml/level3/version1/multi/version1" multi:speciesType="speciesType1" > <multi:listOfSpeciesTypeInstances> <multi:SpeciesTypeInstance id="speciesTypeInstance1" initialAmount="1"> <multi:listOfSelectorReferences> <multi:selectorReference selector="selector1"> </multi:listOfSelectorReferences> </multi:speciesTypeInstance> </multi:listOfSpeciesTypeInstances> </species> <initialAssignement symbol="species1"> <multi:speciesTypeInstanceChange speciesTypeInstance="speciesTypeInstance1"> <math xmlns="http://www.w3.org/1998/Math/MathML"> <apply> <times/> <ci> x </ci> <ci> y </ci> </apply> </math> </multi:speciesTypeInstanceChange> <math xmlns="http://www.w3.org/1998/Math/MathML" /> </initialAssignement>