SId 

The type SId is used throughout SBML as the
type of the 'id' attributes on model elements.

Simple Type Information

Used By

Attributes AssignmentRule/@variable, Compartment/@compartmentType, Compartment/@id, Compartment/@outside, CompartmentType/@id, Event/@id, EventAssignment/@variable, FunctionDefinition/@id, InitialAssignment/@symbol, Model/@id, Parameter/@id, RateRule/@variable, Reaction/@id, SimpleSpeciesReference/@id, SimpleSpeciesReference/@species, Species/@compartment, Species/@id, Species/@speciesType, SpeciesType/@id

Source 

<xsd:simpleType name="SId">
  <xsd:annotation>
    <xsd:documentation>The type SId is used throughout SBML as the type of the 'id' attributes on model elements.</xsd:documentation>
  </xsd:annotation>
  <xsd:restriction base="xsd:string">
    <xsd:pattern value="(_|[a-z]|[A-Z])(_|[a-z]|[A-Z]|[0-9])*"/>
  </xsd:restriction>
</xsd:simpleType>