This base type is very interesting due to its broad polysemy. It is based on the Latin etymon flōs, whose basic meanings 'flower' and 'blossom' are metonymically related and have given rise to numerous metaphorical and further metonymic meanings.
Derived from 'blossom', flōs often denotes THE BEST, MOST BEAUTIFUL PART OF A THING, as in Latin flos aetatis 'the blossom of years, youthful vigour, fullness of youth' (cf. Georges, under flōs), an expression which has been preserved in Romance languages (as in fra. la fleur de l'âge 'the youth'; cf. FEW, 3, 630-638, under flōs). Similarly motivated are fra. fleur de la farine 'la partie la plus fine de la farine', ita. fior della farina, Engadine flur d'farina or gsw. (Swiss German) Blume (cf. FEW, loc. cit.). Meanings having to do with the SURFACE, the HIGHEST POINT of things, as in fro., frm. à fleur de 'à la surface, au niveau de', can be explained as originating from blossom.
Both semantic dimensions ('good' and 'on top') may mutually motivate the designation of the concept RAHM, which already developed in Latin (flos lactis 'cream') and is still well documented in the study area today (cf. also ita. fior di latte 'cream'). Consequently, verbs such as fra. défleurer or Neo-Occitan sanflurá, sonflurá 'to skim' can be easily interpreted (cf. FEW, loc. cit.).