question

I've started to learning T-SQL because I want to get to know ETL tools that I'm using.

Below you can see query from stored procedere that I don't understand in 100%.

I know that it's used to transform data from raw table to output table. I see also that join is used to lookup columns from other tables. But I don't understand part with:

SET
		R.[Business Posting Group Description] =
			CASE
				WHEN (S1.[~Key] IS NOT NULL)
					THEN S1.[~Value]
				ELSE R.[Business Posting Group Description]
			END

Why we use it? Why we have to create [~Key] and [~Value] columns? Can someone explain order of particular steps in this query to me? Thanks in advance.

Full procedure below:

UPDATE R
	SET
		R.[Business Posting Group Description] =
			CASE
				WHEN (S1.[~Key] IS NOT NULL)
					THEN S1.[~Value]
				ELSE R.[Business Posting Group Description]
			END
		, R.[Country Name] =
			CASE
				WHEN (S2.[~Key] IS NOT NULL)
					THEN S2.[~Value]
				ELSE R.[Country Name]
			END
		, R.[Salesperson Name] =
			CASE
				WHEN (S3.[~Key] IS NOT NULL)
					THEN S3.[~Value]
				ELSE R.[Salesperson Name]
			END
	FROM [dbo].[Nav_dbo_Customer_R] R
	INNER JOIN [dbo].[Nav_dbo_Customer_T] T ON
		T.[DW_Id] = R.[DW_Id]
	LEFT OUTER JOIN
	(
		SELECT
			MAX(S.[Description]) AS [~Value]
			, S.[DW_Account] AS [~Key]
			, S.[Code]
		FROM [dbo].[Nav_dbo_Gen. Business Posting Group_V] S
		GROUP BY
			S.[DW_Account]
			, S.[Code]
	) S1 ON
		S1.[~Key] = T.[DW_Account]
		AND S1.[Code] = T.[Gen. Bus. Posting Group]
	LEFT OUTER JOIN
	(
		SELECT
			MAX(S.[Name]) AS [~Value]
			, S.[DW_Account] AS [~Key]
			, S.[Code]
		FROM [dbo].[Nav_dbo_Country/Region_V] S
		GROUP BY
			S.[DW_Account]
			, S.[Code]
	) S2 ON
		S2.[~Key] = T.[DW_Account]
		AND S2.[Code] = T.[Country/Region Code]
	LEFT OUTER JOIN
	(
		SELECT
			MAX(S.[Name]) AS [~Value]
			, S.[DW_Account] AS [~Key]
			, S.[Code]
		FROM [dbo].[Nav_dbo_Salesperson/Purchaser_V] S
		GROUP BY
			S.[DW_Account]
			, S.[Code]
	) S3 ON
		S3.[~Key] = T.[DW_Account]
		AND S3.[Code] = T.[Salesperson Code]