The explanation includes different elements which must be taken into account.
It is in the first place to be borne in mind that the word expedit does not constitute an obligation, but a suggestion that refers to the construction of the altar *a pariete sejunctum *[detached from the wall] and to the celebration versus populum [toward the people]. The clause ubi possibile sit [where it is possible] refers to different elements, as, for example, the topography of the place, the availability of space, the artistic value of the existing altar, the sensibility of the people participating in the celebrations in a particular church, etc. It reaffirms that the position toward the assembly seems more convenient inasmuch as it makes communication easier (Cf. the editorial in *Notitiae *29 [1993] 245-249), without excluding, however, the other possibility.