This native is used to attribute the SRL that BeginSrl is going to load. This is usually used for 'in-game' cinematics (not cutscenes but camera stuff) instead of SetFocusArea because it loads a specific area of the map which is pretty useful when the camera moves from distant areas.
For instance, GTA:O opening cutscene.
https://pastebin.com/2EeKVeLA : a list of SRL found in srllist.meta
https://pastebin.com/zd9XYUWY : here is the content of a SRL file opened with codewalker.