PAL

Ptolemaeus Arabus et Latinus

_ (the underscore) is the placeholder for exactly one character.
% (the percent sign) is the placeholder for no, one or more than one character.
%% (two percent signs) is the placeholder for no, one or more than one character, but not for blank space (so that a search ends at word boundaries).

At the beginning and at the end, these placeholders are superfluous.

ὑποκείμενον ἐπίπεδον‘foundation-plane’

Greek

References: Alm. Heiberg: I.12, 67:2-3
Translated into:
Arabic: البسيط [Alm. MS Tunis: ]
Arabic: الأرض [Alm. MS Leiden: f. 10v]
Modern translations:
English: foundation-plane [Alm. Toomer: 63]