An !ELEMENT DTD declaration is a way to specify DTD constraints for an XML Element.
Single Elements
<!ELEMENT name (#CDATA)> A DTD schema for an XML element name with CDATA.
<!ELEMENT coordinates (x,y) #REQUIRED>
<!ELEMENT x (#CDATA)>
<!ELEMENT y (#CDATA)> A DTD schema for an XML element coordinates which must have the x and y elements inside
Multiple Elements
<!ELEMENT seats (seat+)> A DTD schema for an XML element seats which can have at least one seat as a child
<!ELEMENT seats (seat*)> A DTD schema for an XML element seats which can have any amount of seat elements - i.e. , and are all valid examples.
<!ELEMENT seats (seat?)> A DTD schema for an XML element seats which can have either or child(ren).
Union Elements
<!ELEMENT phone (android|iphone)> A DTD schema for an XML element phone which can have either android or iphone as it’s child.