TSQL - make a literal float value

is that literal a floating point literal? Or is it really a decimal literal that gets converted later?

If my literal is not a floating point literal, what is the syntax for making a floating point literal?

The 0.867153569942739 literal in SQL Server is a decimal type, not float. The engine automatically picks appropriate scale and precision to represent the given literal.

To write a literal of the float type you should use the scientific notation, like this: 0.867153569942739E0

This is documented in Constants (Transact-SQL)

decimal constants

decimal constants are represented by a string of numbers that are not enclosed in quotation marks and contain a decimal point. The following are examples of decimal constants:

1894.1204  
2.0  

float and real constants

float and real constants are represented by using scientific notation. The following are examples of float or real values:

101.5E5  
0.5E-2

The sp_describe_first_result_set can tell us the types of columns

EXEC sp_describe_first_result_set N'SELECT 0.867153569942739, 0.867153569942739E0'

It returns numeric(15,15) for the first column and float for the second.

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If your column RandomGrouping is indexed, it is much more efficient to use a float literal, because when you wrap RandomGrouping in convert(), an index can't be used.

The following query will use an index:

SELECT *
FROM MyTable
WHERE RandomGrouping = 0.867153569942739E0

The following query will not use an index:

SELECT *
FROM MyTable
WHERE convert(binary(8), RandomGrouping) = @Y