In short
Construction servitude is the right established over a unit in a building that is not yet complete. Freehold title is the full ownership right established once the building is finished and the occupancy permit has been obtained.
A unit bought off-plan is usually first titled as construction servitude. That deed is valid and transferable, but it also shows the building is not yet finished. Once the building is completed and the occupancy permit obtained, the servitude is converted into freehold title.
The difference between the two is not only legal but practical. Banks may apply different conditions when lending against a servitude title. In resale, a unit with freehold title is more liquid than one with servitude title; there is less uncertainty in the buyer's eyes.
Why it matters to an investor
The question to ask when buying off-plan is this: when is the servitude established, and under what condition and whose responsibility does conversion to freehold happen. When that is not answered clearly in the contract, long uncertainties can follow handover.