Plasma Osmolality: The total solute concentration within a given fluid compartment (Mosm/kg).
Hence it is NOT dependent on temperature and pressure like OsmolaRity. See image.
An effective osmole is one that is UNABLE to cross from the Extracellular fluid (ECF) to the Intracellular fluid (ICF). Therefore it will generate an oncotic force that draws fluid across a membrane.
Effective osmoles include: Na+ and Glucose.
An ineffective osmole will contribute to total plasma osmolality but because it can freely move from the ECF to ICF, it generates no oncotic pressure. A classic example of an ineffective osmole is Urea.
NOTE: Osmolality may be increased in the setting of increased Urea (BUN); but the tonicity will not change because the increased Urea will freely equilibrate between the ICF and ECF.
Bhave G, Neilson EG. Body Fluid Dynamics: Back to the Future. Journal of the American Society of Nephrology : JASN. 2011;22(12):2166-2181. doi:10.1681/ASN.2011080865.