Tscar, "Mortar Cements" as Spec'ed by ASTM C 1329 if mixed with ASTM graded sand will meet certain test minimuns.... Note creation of "mortar cements " as subset of existing 'masonry cements'.
Standard takes legacy commodity types of premixed masonry cements dividing them by their weight/Cubic foot bag, and declaring 28 liters ~ equal to 1 cubic foot. M,S,N mortar cements divided by their weights/cubic foot... that matches current products offered by lafarge, hoclim, etc......
Some preblended mixes made with more limestone dust then hydrated/hydralic lime won't meet the tensile/flexure hoops of ASTM C1329. This is where C 1329 separates the sheep from the goats.
Use of mortars that will yield higher tensile strengths(ASTM C 1329 compliant) allow engineers to use some of a masonry's actual ability to resist bending and negative loads without failure--no more discounting any ability to resist pulling apart during design and code compliance phases, equalling cheaper/competitive masonry walls.
Nearly all commerical work I build now requires PCL type Mortars to achieve the same goal, fema shelters, or any public building that is considered worthy of being more disaster resistant.