There are slightly different architectures:
1) Each ADC has its own dedicated sample memory. There is usually one ADC per channel pair, hence we get two ADCs and twice the memory on 4 channel instruments. This concept has been used on SDS1000X, SDS1000X-E, SDS2000X (Plus, HD), SDS2000X-E and SDS5000X.
2) There are multiple ADCs, yet just a single shared memory for all channels. Examples for this are SDS3000X HD (2 ADCs, 400 Mpts total) and SDS6000A / SDS6204 H12 Pro (4 ADCs, 500 Mpts total).
3) As 2), but the sample max. rate is shared between all channels as well, as we can see in the new SDS800/1000X HD (2 ADCs, 50/100 Mpts total, 2 GSa/s total).
The maximum memory (= max. record length) can be a somewhat confusing specification, because it only really makes sense together with the specification of the max. sample rate.
In most cases, we are very aware of the bandwidth of our scope, and what we really want to know is how long can we capture at full sample rate, in other words: how slow a time base we can choose without losing any detail.
If we have a DSO with a maximum of just 100 MSa/s (which limits the real-time bandwidth to <40 MHz), then 1 Mpts of memory allows us to capture as slow as 1 ms/div (for a total record length of 10 ms) while still keeping the initial (maximum) sample rate.
On a DSO with max. 4 GSa/s, we need 40 Mpts of memory for the same capture length – but now >1 GHz real-time bandwidth would be possible.
We could say that e.g. in dual-channel mode, the 250 Mpts on a 2 GHz SDS6204 are actually much less deep a memory than the 200 Mpts we can get on the SDS2000X (Plus or HD). Because the latter allows us to capture a 100 ms long record at 10 ms/div and full sample rate of 2 GSa/s, whereas the SDS6000 limits us to just 50 ms record length at 5 ms/div and 5 GSa/s.
what I don't know is what the situation is with the SDS3000X HD or SDS2000X HD with the total segmented memory. Some devices have more memory available with segmented memory. How much segmented memory is available on the SDS2000X HD?
On the SDS2000X HD, there is no difference between max. record length and max. segmented memory.
Only the SDS1000X-E with its comparably short max. record length (14 Mpts per ADC) had significantly more segmented memory (up to 56 Mpts per ADC). Other instruments might have slightly more, such as the SDS824X HD, which can use up to 118 Mpts segmented memory, even though the max. record length is limited to 100 Mpts.
EDIT: Shared memory
and sample rate models added.