All really good watercolor paper has a sizing, it is this glue on the surface of the paper that enables an absorbent watercolor paper, a paper with sizing needs less glue in the pulp which makes the paper softer and more porous. You can read more about this here.
Sometimes, however, the manufacturer fails to glue the entire paper. It may be a small spot unsized paper in a corner, or a large area in the middle of the paper. It has even happened to me that an entire paper has been without sizing. If you have ever encountered this, you know that it is not possible to paint on the part of the paper that does not have sizing. It is like a blotting paper, all liquid is sucked into the paper immediately and there are dark spots when it dries. Wet in wet is impossible, flat washes are impossible, everything is impossible on such a paper. It can not even be used for color samples or as test paper. It’s just a waste.
You can not see that a paper lacks sizing. It is not noticeable until you paint on it. Sometimes it can happen that one of my students, who has spent a lot of time and effort preparing a drawing at home, discovers at the time of the course that the paper cannot be painted on. It has happened more than once. I myself do not have that problem because I always stretch my paper. When I wet a piece of paper, it is clear if part of it, or maybe the whole, would lack sizing. That surface becomes darker and a little transparent.
I myself have encountered this mainly with Saunders & Waterford but also Canson Heritage and Arches have had this problem, even the expensive and handmade Lessebo I have encountered paper without sizing.
If you encounter such a paper, try to complain to the store where you purchased it, you will be met with complete incomprehension. They do not understand what you are talking about “what, sizing that is missing?”, “I have never heard of glue on paper”, “sizing? What is it”. Once one of my students had bought a whole package (10 whole sheets) Saunders & Waterford which all lacked sizing on the surface, many attempts resulted in the whole package having to be discarded. She contacted the store, but they did not understand what she was talking about, I tried to help but it did not go well either, they did not understand. It was simply not possible to explain the problem in a way that they understood.
What do we learn from this? I have no idea! Maybe it would be good if stores knew something about the goods they sell.