subtracto is incorrectly written and it should be written as "substrate or substrate" being its meaning:
Latin Etymology: substratum, past participle of substerno which means spread, cover, cover: " what is extended under 34. It is the layer or level which is below another or others constituting its basis, in which the other levels are supported. This word is applicable to all the semantic fields; linguists, gardeners, geologists, chemists, biologists use it...
