The data requirements are stated in the Delegated Regulation, Article 4. For your convenience I provide an extract of this regulation regarding the data requirements below:
*for the road transport, the standards defined in Article 4 of Delegated Regulation (EU) 2015/962;
*for other transport modes, the use of one of the following standards and technical specifications: NeTEx CEN/TS 16614 and subsequent versions, technical documents defined in Regulation (EU) No 454/2011 and subsequent versions, technical documents elaborated by IATA or any machine-readable format fully compatible and interoperable with those standards and technical specifications;
*for the spatial network the requirements defined in Article 7 of Directive 2007/2/EC.
Apart from the rules stated above, the organizations are indeed free to choose their data format, structure and publication protocols. They are, however, required to provide the metadata and data in a machine readable format. A Belgian NeTEx profile is currently under construction and will be communicated to all stakeholders in the near future.