The jurists distinguished between, at the very least, two kinds of law, ius gentium and ius civile. The former, as defined by the jurist Gaius, consisted of the law as understood and followed universally by all peoples everywhere. The latter, therefore, was that body of law that only applied to a specific city. Both types of law, as far as the jurists were interested, were civil law, as criminal law had ceased to be a serious object of legal study by the time of the Empire, since the emperor typically decided criminal justice. However, the same type of thinking would have applied more or less to criminal and what we would call "constitutional" law (whether the latter existed even in nascent form in Roman thought is a controversial subject). Foreign cities, for example, were understood as having their own res publicae. The jurist Ulpian used a different division, which became more common and which is the one used by those modern states whose legal codes derive from Roman law. Ulpian divided law into ius naturae (often misunderstood as meaning "natural rights," which it doesn't), ius gentium, and ius civile. The latter two in Ulpian's definition are more or less the same as in Gaius', but ius naturae consists of that law which applies to animals as well as humans. Ulpian's big example for this, which appears repeatedly, is slavery. In the Roman world slavery was a universal condition, and all societies kept people enslaved. Therefore slavery was part of the ius gentium. But, argues Ulpian, by the ius naturae the natural condition of all living things at birth is freedom, and though people can be born enslaved that's because in this case ius gentium overrides the ius naturae due to social habits.

What the tripartite division of law, which was never a precisely formal structure but was one that Roman lawyers, judges, and jurists understood in some form, means is that the Empire was explicitly multilegal, existing in what Clifford Ando calls a state of "legal pluralism." The law of the city of Rome applied to the city of Rome and therefore all of its citizens, many of whom lived in far-flung parts of the Empire and had never even been to Rome, and outside that category the iura civilia of whatever other city was in question applied. Most of the time this was not an issue. A thief who was caught at Caesarea was tried at Caesarea by the laws of Caesarea, and a thief who was caught at Antioch was tried at Antioch under the laws of Antioch. Issues only arose when conflict occurred between people of individual cities. In a dispute between two provincials from different cities this wouldn't have been too much of a problem necessarily. If no clear ruling could be judged then the matter might be referred to the provincial governor, who might even then pass it on to the emperor if he was really unsure what to do. Pliny's letters to Trajan are full of problems like this. Typically speaking Roman authorities were not terribly concerned about local courts, and when local matters were referred to Roman judgment the Romans were usually left to interpret the dispute by either applying ius civile or referring to their interpretation of whatever part of ius gentium might apply to the case. The jurists are full of such cases.

No, the real problem occurred when Roman citizens were involved. The Romans judged Roman law to be above the laws of their subject cities, and even of the free cities like those in Greece. That meant that when a Roman citizen was brought up in a legal dispute with a provincial, the Roman citizen was at least expected to have the right to be tried by Roman law. That's what's going on with Paul in Acts. Although Fergus Millar has pointed out a lot of the issues with the story (specifically, Paul's appeal to the emperor does not seem plausible based on what we know), nonetheless Paul's request that he not be forced to suffer the judgment of a Jewish court rather than a Roman court seems to be a typical one. Already back in the Republic it was found necessary to have special praetorian courts set up to deal with disputes between Romans and provincials. The actual situation is much more complicated than just assuming that Roman citizens always got their way--just look at Verres, whose big crime was extorting Roman citizens in Sicily, but who was also accused of treating provincials unlawfully--but at the very least disputes between Roman citizens and provincials required the referral to a separate legal structure, one that tended to but did not necessarily favor the Roman citizen.

The further difficulty of the Constitutio Antoniniana complicated things. It's a matter of great controversy whether the CA actually really did anything, and whom it enfranchised (free persons living in the Empire at the time of its passage? Otherwise how do we justify the continued use of the term peregrini in the later Empire?), but Clifford Ando has quite persuasively argued that in purely legal terms at least the CA had great effects. The Codex was put together from a body of Roman law that extended across centuries and very different conditions. There are preserved in it all kinds of archaisms that look perfectly normal to us, but that would have been completely bizarre to a Roman of the fifth century. For example, much of the stuff dealing with citizenship in the Codex would have been totally antiquated by the extension of citizenship in 212. The Codex refers all the time to Latin status or even Junian Latins, quasi-citizenship categories that had more or less ceased to exist by 212. Ando identified a dizzying number of post-Antonine rescripts in the Digest and the Codex, most of which were directed at provincial towns and most of which more or less just restated what was already true, allowing local traditions to stand, only now in explicitly Roman terms rather than as part of ius gentium or the particular ius civile of that town. As such, by the time of the compilation of the Codex, Roman law had simultaneously become universal while also absorbing and codifying the specific, a feature that Ando argues was a somewhat counterintuitive imperialist reading, reinforcing empire by incorporating and redefining the local.