The patindex is checking for two chars _ and space

charindex('_', @a) will return position of first _ char

patindex('%[ _]%', @a) will return position of EITHER _ or space

charindex('_', @a) and patindex('%[_]%', @a) would give the same result (ie no space)

if checking for presence of _ and space then

CHARINDEX('_', @a) > 0 AND CHARINDEX(' ', @a) > 0

would be the same as

PATINDEX('%[ _]%', @a) > 0

and it would get worse if you wanted to check for the letters a to f as you would have to OR 6 CHARINDEX

or use PATINDEX('%[a-f]%', @a)