An extension is compounded of this :

• a name
• a code
• a zip archive, containing
  • at least one module
  • an extension.xml acting as a glue between the modules (if not present the extension is not compatible with iTop 2.4)
    • when upgrading iTop to 2.4+, the iTop will contain the “incompatible version” it will result in an errored list of extensions (each module composing the extension will bee seen as an autonomous extension). The only way to restore a proper display is to upgrade the extension and remove the incompatible version
    • in order to have a nice UI, iTop know the list of combodo's incompatible extensions and nicely display them using a mapping table
• datas into the store
  • extension's data :
    • code
    • title
    • short desc. (used by the listings)
    • long desc. (used in the detail)
    • image (it should be a photo)
    • screenshots (used in the detail)
    • link to the documentation
    • is it a free extension?
    • is it a stand alone extension
    • tags associated
  • extension versions' data :
    • title
    • version
    • short desc. of this version
    • release date
    • extension's zip file
    • the price of this version (it has no effect if the product is free)
    • iTop version min
    • iTop version max
    • access restriction (is it downloadable or only installable by iTop hub)

Note : all the desc. have to be written using markdown syntax, you can test the rendering of your texts here : PHP markdown parser.