If you have seen the condition I put on the selected feature expression table, obviously I can put "Area" = 643 instead and this will select exactly and only the lots that have the area of 643m2. But if I do this I won't have much options as this number is too specific (in this case it only selected 2 features) and some area might not able to meet the condition and therefore, it won't select anything on the map. So using the tolerance condition, it enables me to have more options on the lot I selected. This is only particular to my condition and what I want to select on my map, you can put whatever condition you want on the expression table, with attention to the language it reads.

Here is more information about the Expression tool on QGIS and all the conditions language you can explore: https://docs.qgis.org/2.8/en/docs/use...

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