Well, it’s a pedantic point, but it’s actually a rescript and not an edict. It’s a bit hard to find, but Oliver Nicholson wrote a brief piece intended to explain this point to undergraduates: “ Laws ending persecution of Christians in the Roman Empire: translation of and commentary on Edict of Galerius and Letter of Licinius”. Without getting into the weeds of Roman law, a subject near and dear to my heart, there were essentially three forms of imperial legislation in the Roman Empire. You have edicts, which. Theoretically the Roman emperor could enact law by his word, but this is complicated in practice. So you have three types of imperial law. You could have a formal decree, which is a pronouncement made by the emperor that had the force of law empire wide. A rescript is a response to a query or petition form an official (usually) in imperial service. This response carried the weight of law and could often have the effect of law throughout the empire. Finally you have mandata, which are commands, usually to direct the actions of specific officials.

To place this in context, the edict of Milan exists in two sources. Lactantius and a later Greek translation in Eusebius. Essentially, the edict of Milan is actually a letter bearing the weight of law sent from the court of licinius to that of Constantine, confirming those terms they had agreed earlier. To clarify, this was not a command issued from by the office of the quaestor of the sacred palace, which would be an edict. This is a letter confirming terms that two claimants to the imperial crown had agreed upon. As Nicholson notes in the piece notes above, persecution of the Christians had ended in the west already. You can read the piece here: https://archive.org/details/milestonedocumen0000unse/page/298/mode/2up. It requires a free log in and you check it out for a limited amount of time, but it is meant to introduce students to reading texts, so you can probably breeze through it and find it interesting.