Those are misleading. They are saying it's both Lexa and Polaris. The cores/shaders are the most important to look at. If it doesn't have 640+ cores/shaders/stream processors, then don't buy one. To be more specific, it has to be Polaris 21 based.
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Not a single Polaris 21 AMD RX550 or 560 has
512 shaders in it. If you see 512 then it is a Lexa card.
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Sapphire quietly introduced a slightly bolstered
Radeon RX 550 graphics card that's based on the "Polaris 21" silicon, rather than the Polaris 20 "Lexa" silicon, called simply the Pulse Radeon RX 550 2GD5/4GD5 (model: 11268-16 for 2 GB and 11268-15 for 4 GB). Consumers should pay attention to the model number.
"Polaris 21" is the same chip AMD bases the RX 560 on. This particular card has 10 out of 16 compute units physically present on the chip, which translates to 640 stream processors, higher than the 512 stream processors the RX 550 is originally endowed with.
For the Sapphire RX550 look for this title:
PULSE RX 550 4G G5 640SP (SP or stream processors are also called shaders)