Being (ens in Latin) is a transcendental, not confinable to any single category, and a common tradition takes there to be five primary transcendental attributes of being that apply to every being in some way. The medievals used the mnemonic reubau to capture them all: res, ens, unum, bonum, aliquid, verum. (If you wanted an English mnemonic to remember the Latin words, you could use bureau instead.) Ens, of course, is being, unum is one, bonum is good, and verum is true. The usual translations for res and aliquid are 'thing' and 'something', respectively, which is how we would usually translate them in other contexts. But in this context, this is probably wrong.
Take Aquinas's account of the six. Being (ens) is first and most fundamental. We can then add the other attributes by considering being absolutely or in relation to another. Good (bonum) and true (verum) are being relative to another, good as being relative to desiring and true as being relative to knowing. These are both, however, based on correspondence -- desiring and knowing are acts that can in principle cover any kind of being. There is a different way in which being can be considered relative to another, and that is by being distinguished from another. This is aliquid, which Aquinas (and most other medieval scholastics) relates to aliud quid, more or less 'another what', i.e., something that is other than what something else is. Note that it's quite important that aliquid convey the notion of distinction or otherness, and that aliquid be related to aliud. Quite clearly neither of these are captured by 'something' as a translation. Something is aliquid insofar as it is distinct from something else.
If we take being absolutely, in itself, we can do so affirmatively or negatively. If we take it negatively, we get one (unum), that is being undivided. Res is what we get when we take being in itself affirmatively, that is, in terms of its positive content. Aquinas follows Avicenna in taking this transcendental to capture the essence or quiddity (whatness) of being. When res is a transcendental attribute of being, it is supposed to capture the quidditas, the whatness, of a thing. Now, the English word 'thing' is very flexible, which is why it's able to keep up with the very flexible res, but it does not capture the essential element here, which is the notion of positive content. And whenever you look at important uses of res in Latin, they always convey something more robust than the English word 'thing' can convey. 'Thing' captures the generality, but not the suggestion of content, the idea that we are talking about something, however generally, in a way that emphasizes what it is. Res publica is not so bland and vague as 'the public thing' makes it sound; it indicates the very content that is public, the substantively public, which is why you need something like 'the common weal' to capture it. The title of the encyclical, Rerum novarum, could be translated as 'of new things', but this anemic translation does not convey the point. It indicates the very substance of things being innovated; the correct translation of the encyclical is more like "Of Revolution(s)" or "Of Upheavals". (The first words of the encyclical, from which the title comes, are Rerum novarum semel excitata cupidine, which I would crudely translate as something like 'the now-stimulated craving for upheavals'.) Not just 'new things' but substantive novelties. And so it is with other things. Res emphasizes the very character or nature that makes it whatever it is. 'Thing', while general enough, is so general as to lose this, the key element that makes res an important concept.
Obviously we have in English no exact correspondence to res and to aliquid. But I would suggest that we would at least get closer if we translated, in discussion of transcendentals, aliquid as 'another', and res, not aliquid, as 'something'. 'Another' has the connection to otherness that aliquid has. And 'something', through the 'some', comes closer to capturing a positive notion of content. ('Substance', in the broad sense, would work better in some ways, but obviously it's always already being used in a narrower sense in these discussions from something that is not transcendental.) Possibly there are better translations, but it's certainly the case that 'thing' for res and 'something' for aliquid don't really cut it.
