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Disclaimer: I am not an expert on Roman Law so if what I say here is gibberish, please facitote me certiorem. Number two, this article could be a lot shorter and to the point, but that’s not the point. Happy reading!

In the previous article I mentioned that I could talk about what ius is because some people translate it as right, others as law, (yet others as soup or juice), but to the Romans, ius was maybe both. I referred to it as “a collection of laws” before.
A better way to think about ius is to imagine it as a responsibility. Now, of course, if you’re responsible for something it also means you have authority over it, so a ius is as much a right as it is an obligation, hence the phrases sui iuris (acting on your own right) and alieni iuris (acting on behalf of someone with authority over you, like when you, a citizen, are called to arms). It is law in its broadest sense.
If this all seems very abstract to you, it’s because from a viewpoint of law as a science, it is. From a practical perspective, it’s not that hard to understand. Let’s first look at what the Romans had to say about it:

Omnes populi, qui legibus et moribus reguntur, partim suo proprio, partim communi omnium hominum iure utuntur: Nam quod quisque populus ipse sibi ius constituit, id ipsius proprium est vocaturque ius civile, quasi ius proprium civitatis; quod vero naturalis ratio inter omnes homines constituit, id apud omnes populos peraeque custoditur vocaturque ius gentium, quasi quo iure omnes gentes utuntur. Populus itaque Romanus partim suo proprio, partim communi omnium hominum iure utitur. – Gaius (we don’t know his full name)

Now what’s interesting here is that the ius gentium is the ius common to all men, according to the Romans. It’s natural law. Therefore the populus Romanus uses partly its own ius (the ius civile), partly the ius common to everyone. Gaius also calls them ius civile and ius naturale for this reason.
But to get back to the subject, we have two pretty good definitions for ius, both of which apply at the same time. The first is ius est norma agendi, almost a “rule of conduct”, and ius est facultas agendi, roughly a “licence to act.”
They had many of these iura, and a lot of it depended on your social status. Essentially, you could be a Roman citizen, having the ius civile, a Latin (but not a Roman citizen), having the ius Latii, a foreigner (peregrinus), or a slave (servus/a). A servus would not really have rights but at the same time they wouldn’t have all the responsibilities that a Roman citizen would either.
The way Rome treated other Latin cities for a while was to make an alliance with them and give them the ius Latii, which is a bit of a misnomer really since the exact rights were dependant on the alliance itself and could be anything from the ius commercii (the right to trade), which doesn’t “let them trade” but “lets them trade with Romans on the same terms as Romans trade with each other, using the same contracts”, to even the ius civitatis mutandae, a ius that gives you the right to become a Roman citizen. Fun fact: this is the way the word Latinitas was used by jurists, referring to the state of having the ius Latii.
Now why was the ius Latii useful for these Latin cities? Because after essentially being conquered by the Romans, their area didn’t become an ager Romanus and so the Romans couldn’t buy up their lands. Their internal politics was also left untouched at the time—later, of course, as Rome grew, so did their politics and laws change—concerning their external politics, well, they had to serve Rome fully. Also, they could perform Roman rites and sue people following Roman custom, and if they freed a slave, that slave also received the ius Latii.
But how does a ius tell us all of this? Let me refer to Gaius again here:

Constant autem iura populi Romani ex legibus, plebiscitis, senatus consultis, constitutionibus principum, edictis eorum, qui ius edicendi habent, responsis prudentium. – Gaius

So we have leges (laws), and plebiscita (decrees of the plebs), senatus consulta (decrees of the Senate), edictis (edicts), and responsa prundentium (some authoritative opinions of leading lawyers), all which can constitute a ius. Fun fact: by the time of Gaius, a lex and a plebiscitum was, from a legal point of view, the same (since the legal differences between patricians and plebeians were done with).
If there was any doubt in what a ius was, now the translation “collection of laws”, or “right”, or “law”, or “responsibility” all make sense. The exact details of what a ius meant was decided and influenced by all of the above, and ius is really an umbrella of all of these.
Credits where they’re due: I know everything from a good friend of mine specialising in Roman Law. While reading Latin works, one will inevitably meet with things they would not understand without the cultural context, which often has to do with law or how Roman society was structured in general. I have a good friend back from high school who just knows everything so he is my go-to man whenever it comes to Roman law and Roman society. 
If you are interested in more Roman Law, do let me know and I will divulge all that I know.