See 2016 version of this map at same scale here: https://censusmapper.ca/maps/696

This is the ratio of Duplexes to Single Detached Houses, so a neighbourhood with a low number would have relatively fewer of it's housing stock as duplexes.

Per census definitions, a detached house with an auxiliary unit, such as a basement suite, is listed as a duplex.

In a neighborhood with only detached houses, some of which contain basement suites, a ratio of '1' would imply that every third house has a basement suite (Single+Single+(Main+Basement)). A ratio of 2 would imply that every other house has a basement suite (Single+(Main+Basement))

The 'Blue zone' with large numbers of duplexes/basement units seems to be a Lower Mainland phenomenon. Compare to Calgary or Toronto. In Montreal, however, seems to have very few duplexes in single family areas and very few single family houses in duplex areas.