Following you can find several sources of law regarding to the subject invitation to tender.
Utilities Directive (2004/17 EC)
Classical Directive (2004/18 EC)
Defence Directive (2009/81/EC)
Regulation Public passenger transport (1370/2007)
Concession contract Directive (2014/23/EU)
Public procurement Directive (2014/24/EU)
Utilities (water, energy, transport and postal services) Directive (2014/25/EU)
More sources of law on http://eur-lex.europa.eu.
