In short
The handover protocol is the document recording that a unit has been delivered to the buyer and the condition it was in at that moment. Once signed, the ground for objecting to visible defects not written into it narrows.
A proper handover is done by walking the apartment with a checklist in hand. Doors, windows, sockets and switches, plumbing, heating, kitchen and bathroom fittings are each tested. Every defect seen is written into the protocol, leaving nothing to assumption.
The phrase "received unconditionally" is the sentence a buyer should watch most closely. Signing it without listing defects can mean those defects were accepted. Latent defects appearing later are a separate matter and that phrase does not cover them; but a defect visible on handover day and left unwritten becomes hard to come back to.
Why it matters to an investor
Handover day is the last day on which the buyer still holds negotiating power. Once the keys are taken and the protocol signed, the relationship stops being a purchase process and becomes a claims process. The difference is the time you will spend on the same defect.