OK, this is Chicago, the land of conduit, this is a condo, with main breaker at meter in close with sub panel with a main breaker in panel, the neutral bus bar on the right side of the panel is floating, the on on the left side is not with a steel strap connection each.
I had to think about this for a minute because your answer seemed obvious at first.
So what would you call that breaker in the subpanel? Panelboards usually come in two versions MLO (main lug only) and MAIN breaker. I would call the breaker at the meter the service disconnect and the breaker in the sub a main breaker.
If the neutral is floating like you said, then then neutrals on the left side are in the wrong location. It looks to me that there is a lug on the right side of that ground strap, for a ground wire, if it were run with the other feeder conductors. It looks like they are using the conduit for the equipment ground.
As an electrician, this stuff is academic to you Robert, and calling this the “main breaker” is quite reasonable. However, for some reason, it seems that many inspectors have a difficult time making the differentiation between “service equipment” and “other equipment.”
Using the term “main” when referencing panels or breakers seems to perpetuate this confusion among inspectors, which is why I suggest using the the terms “service” and “other” when referring to equipment/panels and breakers/disconnects.
Seems to me breaker or not, this is a sub panel and the neutral (grounded conductor) should float. White wires should all be on the right and the bonding jumper(flat cooper strap running behind the bus bars) should be removed from the right bus. The left bus would be reserved for ground wires.
Some jurisdictions require main disconnect to be accessible to the condo occupant, hence second “main breaker” in this panel. ITE panel?
I would still call it the main breaker to the remote distribution panel.
Looks like they mixed neutral and ground at the sub which is a no no since the box is grounded and also bonded to the neutral bar.