(No 1.05.03 product version yet in Mantis, thus I chose 1.05.02).

Seems 1.05.03 added UnitType (https://reference.opcfoundation.org/Core/Part8/v105/docs/6.4.2.1) and QuantityType (https://reference.opcfoundation.org/Core/Part8/v105/docs/6.4.1).

Why there is a Property named 'Description' whose datatype is LocalizedText when an OPC UA Node already do have an Attribute 'Description' (whose DataType is also LocalizedText)?

Technically I guess nothing disallows this in the spec, but it does cause some headaches for code generators (it is very likely a class would have now something like "getDescription" for both the Attribute and the Property, which would collide). Yes, it is sort of the same thing like namespace collisions, however I would wish the core information model wouldn't do that by itself with the well known Attribute names.

The https://reference.opcfoundation.org/Core/Part8/v105/docs/6.4.1 says " The DisplayName Attribute of each instance shall provide the name of the quantity, e.g. “acceleration”, “battery capacity” or “pressure in relation to volume flow rate”. " so why not use the Description Attribute similarly for it's purpose as well?

Since the DataType here is even the same, this would just look like a typo or modelling error. If not, please add text to the spec to explain what is the difference for the Attribute and Property "Description" values both and how they should be filled (differently) here. If yes, please make an errata that removes the Property and advises to use the Attribute for this.