“There is a plurality of meaning in the use of terms related to affordability and affordable housing that can vary across jurisdictions, between the housing and real estate industries and across different stakeholders.”
Housing market structures have evolved independently across countries. Although there is some common housing sector language across countries and between public, NFP and private sectors, the exact meaning and interpretation of terminology varies. 
In particular, the term ‘affordable housing’ may be used to represent multiple housing segments and the scope of each segment varies across markets.
Similarly, what constitutes an ‘affordable’ rent is often ill-defined.
To learn more read Chapter 2 of the INREV paper ‘Housing middle income Europe: the intermediary investment opportunity amid diverse residential market structures.’