There are three levels, and they get mixed up constantly: the company (the legal entity with a company number), the agency (the name the market knows, sometimes several companies together) and the group (the holding or international network above it).
For you the third matters most, for one reason.
Two candidates, one owner
Two agencies competing in your pitch can hang off the same holding. That is not rare: the large international groups own several Belgian agencies that compete with each other on paper.
That does not make their proposals worth less. It does make your pitch less independent than it looks, and it is the axis most often skipped in a selection. Discover it only after the award and you have negotiated against yourself.
Where to look it up
IKAg's factsheets show per agency which group it belongs to, and there is an overview by group with the agencies underneath. Beyond that: the Belgian Official Gazette publishes participations and director changes, and annual accounts in the full schedule list participations.
What the group does and does not mean
It does mean: access to specialists inside the group, international coverage, and the processes large advertisers need.
It does not mean: that the agencies work together. In most groups they are separate businesses with their own targets, and those targets can be opposed.
And it does not mean: a quality judgement. No measurement says a group agency makes better or worse work.
The question to ask
"Are there agencies in this pitch belonging to the same group, and how do you guarantee nothing is shared?" Ask all participants at the same time rather than one of them, otherwise it becomes an accusation instead of a procedure.
See also: network or independent agency and how many agencies to invite.