The nth bit (from the left, starting at 1) of field f is set to
1.
n must be positive, and either have type I or be convertible into
this type. The value of n must not exceed the length (in bits) of
f.
f must have type X . It can be of any length up to the maximum
permitted for data type X.

SY-SUBRC <> 0:

The value of n was greater than the length in bits of f.

Effect

You can use the ... TO g addition to determine the value to
which the bit should be set. g must have the value 1 or
0.

Example

DATA F(2) TYPE X.
SET BIT 1 OF F.
WRITE F.

Output: 8000

BIT_OFFSET_NOT_POSITIVE: n equals or is less than zero.