Same entry in the Synagoge (sigma59) and Photius' Lexicon (sigma169 Theodoridis), and very similarly already in Hesychius (sigma490).
The terminology, of headword and glossing synonym alike, comes from Roman law. Latin selectus, which the present headword directly transliterates, means that the judge was not elected but appointed, by the praetor; he had to be previously enlisted on a list of judges called the album iudicum selectorum. The iudex pedaneus -- a title which lies behind the lexicographers' xamaidikasth/s (xamodikasth/s in Hesychius); cf. LSJ s.v. -- appears, during the course of the Principate, to replace the iudex privatus of the Republic. He was in charge of small private matters.