Various communication models are provided for the exchange of information between the parties involved and the IRS. Greece, pursuant to the agreement it has signed, will follow model 1A. At technical level, the model requires the dispatch of data to the tax authority of Greece and then forwarded to the USA.

Pursuant to the planning, communication between the financial institutions and the tax authority will be made with XML type files. Files for the convenience of all parties will follow the XML 2.0 reporting standard as described in the technical specifications notified by the IRS.

To send the data to the Greek tax authority, only the XML file with the data is required. The file will be accompanied by meta-data and will not be encrypted. The procedures that concern cancellation or amendment of data are still in force (dispatch of corrective or cancellation file)

The following addresses contain the information regarding the format defined by the IRS for data exchange.

https://www.irs.gov/businesses/corporations/fatca-xml-schemas-and-business-rules-for-form-8966

Out of the information included in the page, the links are of immediate interest:  FATCA XML v2.0 
Publication 5124, FATCA XML Schema v2.0 User Guide

As well as the templates:
AmendedNilFIReport
NewNilFIReport
VoidNilFIReport 
OptionalElements

On page:

https://www.irs.gov/businesses/corporations/fatca-ides-technical-faqs all the “Frequently asked questions” concerning the project are gathered here.

The ”Data Format and Structure” is of technical interest.

In the specifications of XML type file, no modifications are expected, for example addition of data, but it is being examined whether it is necessary to change the concepts of some of the existing fields in order to better serve the dispatch of data to the tax authority. As an example, we indicate a hypothetical variation, for example he MessageRefld element should include as the first component the VAT number of the organization sending the data. 

To facilitate the reading of the technical specifications of the scheme (XML 2.0), it should be noted that the “PoolReport” element is not used in the data exchange in the case of Greece.

During the first eexchange, data for years 2014, 2015, 2016 and 2017 must be sent. Each year will be sent on a separate file. The format is the same for all files (XML 2.0) The data included in the records must be at least the data designated by the IRS as necessary for each period. If the file includes information that was not necessary for the filing period in question, the IRS will accept the additional information, but if it finds any errors, the file must be corrected and sent again.Details of the required data per period are included on page  https://www.irs.gov/businesses/corporations/summary-of-fatca-timelines  

At  http://www.aade.gr/en/international-issues/issues-international-tax-content, you can find information on the legislation on International Administrative Support issues.