Difference between datalength () and Len () in SQL Server

Code

Declare   @   Nvarchar ( 20 ),
@ B   Nvarchar ( 20 ),
@ C   Nvarchar ( 20 )

Set   @   =   ' ABC '
Set   @ B   =   ' ABC '
Set   @ C   =   ' ABC '

Select   Len ( @ ) As   [ Len ] , Datalength ( @ ) As   [ Datalength ] -- 3, 6
Select   Len ( @ B ) As   [ Len ] , Datalength ( @ B ) As   [ Datalength ] -- 4,8
Select   Len ( @ C ) As   [ Len ] , Datalength ( @ C ) As   [ Datalength ] -- 3, 8

 

Len () returns the number of characters in the specified string expression, excluding trailing spaces.

Datalength () returns the number of bytes used to represent any expression.

 

However, I also found the following problems:

 

Code

Declare   @ D   Varchar ( 20 ), -- Variable Length, non-UNICODE character data. NThe value range is 1 to 8,000. MaxIndicates that the maximum storage size is 2 ^ 31-1 bytes. The storage size is the actual length of the input data plus two bytes. The length of the input data can be 0 characters
@ E   Nchar ( 20 ),-- NCharacters in length. NThe value must be between 1 and 4,000 (inclusive ). Twice the storage size NBytes.
@ F   Char ( 20 ) -- Fixed length, non-UNICODE character data, length is N Bytes. N The value range is 1 to 8,000. The storage size is N Bytes

Set   @ D   =   ' ABC '
Set   @ E   =   ' ABC '
Set   @ F   =   ' ABC '

Select   Len ( @ D ) As   [ Len ] , Datalength ( @ D ) As   [ Datalength ] -- 3, 3
Select   Len ( @ E ) As   [ Len ] , Datalength ( @ E ) As   [ Datalength ] -- 3, 40
Select   Len ( @ F ) As   [ Len ] , Datalength ( @ F ) As   [ Datalength ] -- 3, 20

 

I am confused about the results returned by datalength for these three types. Please kindly advise. Thank you!