Khodadad Rezakhani:

"Parthian ... was written in a script based on Aramaic and it incorporated heterograms. Our main contemporary sources for studying the Parthian language are the few remaining inscriptions and ostraca from Nisa and Hecatompylos. However, a great portion of our available Parthian material comes from the early Sasanian period in form of Manichaean compositions and pieces such as the Parthian version of Shapur’s inscription, and as such represents a very late stage of this language. Manichaean texts used a special script reportedly created by Mani himself, and it did not use heterograms ...

Heterograms or hozwarish, are Middle Iranian ideograms, written in their original Aramaic form, but read and pronounced in their Parthian (also Middle Persian and Sogdian) forms. For example, the logogram BRA was read pus in Middle Persian, and written as BRY, was read as puhr in Parthian, both meaning 'son'."