“Sociedades bajo matriz” is an extremely weird-sounding expression in Spanish, at least in Spain, but to me the preposition “bajo” has an unequivocal meaning: It conveys the idea of something depending on or subject to something else, in this particular case some companies that depend on another company, which would be the “sociedad matriz”, the parent company or corporation, identified here as “compañía XXX”.

The “sociedad matriz” (but on no account “bajo matriz”) would therefore be a company that owns more than half the shares of other companies, which are then the “sociedades bajo matriz”, its subsidiaries.