The question is somewhat confusing.

The field is given as 'Differential pressure measurement'.
The question is about 'superimposed steam'.
The first line of the ST extract says "Level measurement in a closed container with superimposed steam".
The text that follows 'Sur cette figure, on voit que ...' refers to the relative positions of system components, but there's no mention of steam.

Most of that info is 'red herrings' as far as 'superimposed steam' is concerned. This term simply refers to the fact that if a closed container contains water that is being heated, the space above the water will be occupied by water vapour and/or steam (depending on the temperature and pressure). That will have implications as regards the characteristics of the device used to monitor the water level (which may or may not involve some kind of (differential) pressure gauge).

https://archive.org/stream/traiteelementair02gano/traiteelem...
D'après les lois connues, la quantité de vapeur superposée au liquide augmente très rapidement, et sa densité croit dans le même rapport, au delà de toute limite. D'autre part, la portion demeurée liquide éprouve une dilatation croissante qui finit par dé- passer celle des gaz (expériences de Thilorier). Il est clair que, par l'effet de ces variations inverses de densité, on finit par atteindre une température limite où liquide et vapeur ont la même densité.