Okay, I know I'm thinking too hard about this, but I find it compelling (it doesn't mean it's right though, I know, but it's fun!)
For me, the logic of combining them - particularly via bitwise XOR boolean logic - is because the way we get unknown links is by passing a UA and UP into the thing in the side chamber, thus manufacturing a UL. That chamber needs the two to make the third one so the three items are already intrinsically linked together - the UL is a product of a UA and a UP - what's not known is what operation is being performed.
So then we have the labelling on the new spectrogram, echoing some of the labelling on the UP spectrogram.
If we interpret the labels as binary with least significant bit on the right, then
UA = 001 = 1, UL = 010 = 2, UP = 011 = 3.
Starting with arithmetic, since we know that we need a UA(1) & UP(3) to produce a UL(2), then we just need to figure out a way to combine 1 & 3 to get 2 - which is easy:
UP - UA = UL because 3-1 = 2.
Also:
UP = UL + UA
But it seems a bit odd to have to subtract in one case, but add in another.
This is where bitwise XOR comes in, which also has the added bonus of not having to assign the bit patterns any particular number either - they're just 001, 010 and 011.
So, as we know, to make a UL you need a UA(001) and UP(011). This is a bitwise XOR:
001 ^ 011 = 010
Which then implies that to make a UP(011) you need a UA and a UL:
001 ^ 010 = 011
And to make a UA you need a UP(011) and a UL:
011 ^ 010 = 001
The new spectrogram clearly suggests that there is a functional relationship between all three, with the UL sitting in the middle (and, not least because it's called a 'link'!) - which is why I'm suggesting that ULs are somehow two halves of the UA and UP joined together - providing the bits that neither shares (and then backed up by the fact that the link appears to react to both when dropped in space) - in order to act as a 'bridge' (or 'link') between them - that is, quite literally a definition of XOR.
I'm not saying there's anywhere we can actually manufacture UAs and UPs, just that the fact we can manufacture ULs from UAs and UPs, and the way they're labelled, suggests they're all a combination built from the same set of two distinct pieces in different combinations.
And this actually leads us to be able to make a prediction about whatever '| - -' is, if we are indeed intended to interpret that also as a labelled 'thing':
Whatever it is, is unique: it doesn't have any parts common to UAs, UPs or ULs - and the Unknown Device would appear to fit that bill.