An SE stands for Shader Engine. It is something that has been part of AMD's graphic cards since GCN second gen. Here is the wikipedia entry:
" GCN 2nd generation introduced an entity called "Shader Engine" (SE). A Shader Engine comprises one geometry processor, up to 44 CUs (Hawaii chip), rasterizers, ROPs, and L1 cache."
The PS4 had one single SE with 18 CU running at 800Mhz, while the PS4 Pro had two shader engines with 36 CU running at 911Mhz. When the PS4 Pro was playing original PS4 games that had not received a enhanced patch it disabled one of the SE to emulate the actual hardware of the PS4 so that there wouldn't be any bugs or glitches.
For this generation (NAVI, instead of GCN) AMD is still using shader engines (SE) and at this point all of their graphic cards have either 1 SE or 2 SE, but there are rumors that some of the upcoming big Navi GPUs will have 4 SEs. At this point we are pretty sure that Arden is the Series X GPU and it uses 2 SE with 56 CU running at approximately 1.675 Mhz. For Sony the really old info on Ariel said it used 2 SE with 36 CU, and then the later backwards compatibility testing that was tied to PS5 also showed 2 SE with 36 CU at 800Mhz, 911Mhz, and 2.0 Ghz. This shows that the PS5 will probably use the same method of backwards compatibility, running 1 SE with 18 CU for PS4 games, running 2 SE with 36 CU for PS4 Pro games, and then maybe running 2 SE with 36 CU at 2.0 Ghz for games that have an engine that can take advantage, or maybe for PS5 games.
We don't have any idea if that is what the final chip is though because that leak did not show RT and VRS, while it did show those things for Arden (the XSX GPU). Now that we know PS5 also has RT and VRS it makes the Github leak look like it may have only been testing for BC because it is missing final details. The thought is then that maybe PS5 has 3 CU with 54 CU, and that the Github data only showed backwards compatibility testing.....