Look At It This Way
Bottom of Line (BOL)- Lowest priced and often most basic of any produced goods. Few to nil frills or extras, just basic "get the job done" .
Middle of Line (MOL) - Step up in features and usually quality for the "average" consumer. May have some features of the higher priced lines and or get them as "hand me downs" a product cycle year or so afterwards (for instance originally moisture sensors were a feature of TOL clothes dryers, now they are found on MOL and even some BOL offerings as well).
Top of The Line (TOL)- Best in quality and highest priced of a consumer goods line. Here you find latest trends, inventions, luxury, features and so forth.
Quality can also vary by brands. Miele for instance even at their most basic offerings (BOL) are probably going to be streets ahead of say a Candy or Merloni similar product.
If one uses automobiles as an example BOL cars would be simple basic transport. You get something that moves you from point "A" to point "B" with few if any frills such as power windows, and until recently in the USA automatic transmission.