determining resolution in Lightroom

The best analogy I can come up with, is that of cash money.

There are a certain number of pixels present in, or Raw-convertible from, the file that is imported into Lightroom. This is equivalent to a certain amount of money being paid into someone's bank account - say, $1300.

The imported file also may be marked with a ppi figure. For example, when a camera makes a JPG, the standards require some kind of ppi to be set, even though the camera has no idea what usage the picture will be later put to. Is it intended for art quality printing, or is it intended for a webpage? The camera just sets whatever notional figure it has been programmed to always set.

This import ppi is equivalent to the form in which the money was originally paid into the account - it may have been, as it happens, 13 x $100 bills. Or it may have been 26 x $50 bills. Or it may have been 13,000 10c coins. Maybe we cannot even find out. It may just have been some number in an electronic bank transfer or on a check, with no bills or coins involved at all.

What counts now, is that there is $1300 in a bank account, available to be spent.

So you withdraw some money and spend it: you print, or you export, and you assign whatever ppi you want to that. It is (more or less) a mirror situation of the paying-in, in terms of how important (or not) the ppi setting is. Perhaps you upsample the money into 26 x $100 bills, which would need to be done in Elbonian currency, which has half the face value of normal currency, so that you can get $2600 Elbonian out of $1300 normal without violating the laws of preservation of cash (detail). Or perhaps you just spend $13-worth and don't need to empty the whole account into that payment.

IOW, the limits on what can then be "paid out", have nothing to do with ppi (except as that has affected the quantity of pixels concerned).

You don't necessarily have more money if someone has paid $50 bills into your account, than if someone has paid $10 bills into your account, because it depends how many bills of each type we are talking about. We first need to know the total number of dollars, by multiplying it out.

Or, we need to have some external knowledge which amounts to the same thing - such as, if we saw that there was a bundle of 13 physical bills paid in, we can work out in advance what consequence the actual denomination of those bills is going to have. Similarly, maybe we know that this 300ppi file, and that 1200ppi file, are both scans made from the same 6"x4" artwork, and that tells us (precisely the same in practice, as if we know that one has 2.1MP, and the other has 34.6MP) what the practical limits are likely to be, on how each version can be later used.

Your "a specific ppi at a specific size" is an output question so far as LR is concerned, not an input question. If you asked an accountant about payments in and out of your bank account, the conversation would happen in terms of spendable dollar amounts, not in terms of bills and coins. The same applies in LR, where you can list the pixel count of two images and then compare the two directly. If LR instead told you whether the ppi of each happened to be the same, or different, it would also need to tell you whether the notional page dimensions happened to be the same or different. Unless both figures matched, you would still have to work out and compare the pixel counts (which LR already gives you) in order to compare. If you only refer to partial information, such as the ppi alone, you cannot work anything out and no useful comparison is possible.

RP

The ppi is set or used in another application is irrelevant and is just a tag in the metadata. You set that tag when you export in the Export Menu here: