Siège in this context means "registered office".

Here is an excerpt from a translation that I did for a major retailer in Belgium, which explains what is happening here...

If the Vendor is a company or foreign legal entity and it imports its articles, which are covered by a Demonstration Agreement, into Belgium, then the Vendor must be legally registered in Belgium (including a VAT number).
This relates to the supply of data and the way in which journals and administrative documents are set up as part of the execution of the Demonstration Agreement (VAT, Intrastatt statistics).

The obligations of a Vendor who is in this situation are explained hereafter:

 The Vendor must register with the Belgian VAT administration. To do this, he may contact a tax advisor in Belgium (who can also be the fiduciary).

 The tax advisor or fiduciary applies for a jointly held Belgian VAT number: in both its own name and in the name of the foreign Vendor.

 Any invoice for XXXX issued by the foreign Vendor must be addressed to its tax or fiduciary agent in Belgium. The agent will invoice XXXX by issuing a new invoice or by attaching an additional invoice for the VAT to the Vendor's invoice.

 It is also the tax agent who will communicate to the National Bank of Belgium data relating to flows of goods coming from or going abroad (Intrastatt).

 XXXX does not intervene in any case in the relationship between the foreign Vendor and his tax or fiduciary agent in Belgium.

 XXXX does not authorise any exemptions from this procedure.