Some XSD explain please.

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XSD

Hello. I'm a .NET developer, and I'm working on the IFC format, which is the main format for exchanging data between each stakeholder of a construction project.

Initially this format is described in EXPRESS language and the information was stored in the STEP21 format.
But recently, BuildingSMART, the standard setter, also proposes that IFC data can also be stored in xml (ifcxml) format. It therefore proposes an XSD file to describe as was the case in EXPRESS.

Currently, my job is to create a minimalist relational model to simplify the original model. Having not found a programming library to read the EXPRESS language, I am forced to content myself with the XSD format data, which I do not master at all. But I'm trying. However, there I am before an information that I do not know how to interpret. So i really need your help

This link will show you the same i paste down here

XSD Specification:

<xs:element name="IfcActor" type="ifc:IfcActor" substitutionGroup="ifc:IfcObject" nillable="true"/>
<xs:complexType name="IfcActor">
 <xs:complexContent>
  <xs:extension base="ifc:IfcObject">
   <xs:sequence>
    <xs:element name="TheActor">
     <xs:complexType>
      <xs:group ref="ifc:IfcActorSelect"/>
     </xs:complexType>
    </xs:element>
   </xs:sequence>
  </xs:extension>
 </xs:complexContent>
</xs:complexType>

EXPRESS Specification:

ENTITY IfcActor
	SUPERTYPE OF(IfcOccupant)
	SUBTYPE OF IfcObject;
	
TheActor	: 	IfcActorSelect;
	INVERSE
	
IsActingUpon	: 	SET OF IfcRelAssignsToActor FOR RelatingActor;
END_ENTITY;

As you can see :
- IfcActor inherits IfcObject
- TheActor is an specific attribut of IfcActor. and this is not an array !

So... WHAT THE HELL <xs:sequence> means here ? Please tell me.

What I have tried:

I'm done. i spend two days of work, by searching without result nor comprehension. That's what i have tried.