Grout is a cement or mortar used to fill the gaps between tiles or paving stones after they have been laid, but that is not what this is, and it is for floors rather than (outdoor) paving.

I have taken the liberty of looking up this product. After the paragraph quoted in the question, it says:

"CAMPO DE APLICACIÓN
Mortero especialmente diseñado para la ejecución de soleras, para la posterior colocación de parquet, cerámica, terrazo, etc. Se emplea para pavimentos de nueva elaboración y recrecido de antiguos pavimentos."

I won't quote the URL, in case there are confidentiality questions, but it's clearly the same stuff.

So this is a mortar used to provide a level base on which to lay a floor. That is indeed what "plastón" means:

"La forma de igualar la superficie, de conseguir un plano sobre el que apoyar el solado, es extender una capa de plastón, arena consolidada con cemento y mezclada con agua.
La foto 002 nos muestra la mezcladora de plastón, que, una vez batido, es conducido mediante una manguera hasta el tajo. En la foto 003 vemos el cazo al que llega el plastón, y en la 004 cómo llega el plastón. Los operarios distribuyen la pasta en la foto 005, mientras en la 006 alisan la pasta y la llevan al nivel deseado."
http://www.panoplia.net/textosolados.htm

In English this is normally called floor mortar, or sometimes flooring mortar:

"S-780 FLOOR MORTAR BED MIX"
http://www.summitville.com/pdfs/S-780.pdf

"The first reaction most people have when they see and feel the consistency of mortar used for floors, countertops and benches is “that looks way too dry!” Floor mortar, commonly called deck mud, is probably not what you might expect if you are used to seeing concrete, brick mortar or stucco. All these materials use Portland cement, aggregates and typically water to begin a chemical reaction."
http://www.fcimag.com/CDA/Archives/94b5b318a1cac010VgnVCM100...

"Self-Levelling Overlayment is a premium quality pre-mixed mortar consists of special cements, graded fillers, polymers and chemical additives. When mixed with water, it readily blends to produce a flowable and pumpable flooring mortar."
http://www.cementconnections.com/eng/p2.asp?ItemClass1=SELF-...