The definition.xml file is the definition of your extension, number of dimensions/expressions, text fields etc etc
The properties.qvpp files, comes in a few different names, is the file that defines the properties dialog of your extension.
Normally you do not need to edit the properties dialog as it will be auto generated based on the definitions that you defined in definitions.xml
If you want to construct your own properties dialog you would specify that in definition.xml and then build the dialog on your own using xhtml.
The first is to apologize for my English.
It's simple, in definition.xml file structure or skeleton of our object is constructed and is required.
The Properties.qvpp file (QlikView page properties) is optional, and it format and style properties defined form. It is written in XHTML as you said Alexandre and from QlikView 11 is automatically generated DynProperties.qvpp based on the file Definition.xml.
If you would like to use a custom file properties page, you can use as a base or template the DynProperties.qvpp generated automatically, and indicate in PropertiesPage parameter file ExtensionObject label definition.xml
Sorry, you have forgotten me clarify a nuance. In the form object properties, several leaves with grouped options, one General, one for Dimensions, one for Measurements, ... For in the file definition.xml can say that the properties of specified Dimensions and Measurements, and the rest, as Presentation Title ... Properties.qvpp specified in the file.