While most modern radios are vary accurate in regards to frequency, not all radios are. Our signals get garbled with the transmitting frequency of one radio is not the same as the receiving frequency of the other.
By slightly adjusting the receiver frequency with RIT, you can sometimes get better reception of a station that is transmitting slightly off ...
Modern radios that have RIT and XIT typically also have a button that you can use to listen on your transmit frequency. This means that if you use XIT or RIT, then you can listen on your transmit frequency (rather than on your receive frequency) to check conditions on there.
The typical use case for RIT is if you are calling CQ and you start to get a pileup, ...
RIT means that your VFO frequency will be offset from your dial frequency (usually by a relatively small amount) while you are receiving. XIT means that your VFO frequency will be offset from your dial frequency while you are transmitting.
So a dial frequency of 7125 kHz and a RIT of +1000Hz will accomplish exactly the same thing as a dial frequency of 7126 ...
That was just added by someone who's primary language probably isn't English. What they meant to say is "QPSK31, a mode that is related to PSK31, uses..."
Other than that, the claim "it uses more constellation points, that's a means error-correction coding", is plain wrong.
The error coding used is this, just one sentence later:
In this case, "allied mode" can be replace with "associated mode" or "kindred mode." The phrase indicates that there is a relationship between the two modes, BPSK and QPSK, without specifying the particulars of the relationship. "Allied mode" is not meant to convey or identify any specific technical content.
If you were designing a link budget, EIS is the minimum power required at the receiver, assuming the receiver is an ideal isotropic receiver. The EIS encapsulates the effects of both the receiver's sensitivity and antenna system.
A link budget might look like this:
+/- adjustment due to antenna mismatch
- transmitter feedline loss