In an expression $$a \times 10^b$$;

The order of magnitude of a number expressed in scientific notation as $$a \times 10^b$$ (where $$1 \leq a < 10$$) is defined based on the value of $$a$$. According to the convention used in this context:

If $$a \leq 5$$, then the order of magnitude is $$b$$.
If $$a > 5$$, then the order of magnitude is $$b+1$$.

Therefore, $$b$$ is the order of magnitude when $$a \leq 5$$.

Now, evaluating the options:

Option A states that $$b$$ is the order of magnitude for $$a \geq 5$$. This is incorrect because when $$a \geq 5$$, the order of magnitude is $$b$$ only if $$a \leq 5$$, but for $$a > 5$$, it is $$b+1$$.

Option B states that $$b$$ is the order of magnitude for $$a \leq 5$$. This is correct, as per the convention.

Option C states that $$a$$ is the order of magnitude for $$b \leq 5$$. This is incorrect because the order of magnitude is an exponent (a power of 10), not the coefficient $$a$$.

Option D states that $$b$$ is the order of magnitude for $$5 < a \leq 10$$. This is incorrect because when $$a > 5$$, the order of magnitude is $$b+1$$, not $$b$$.

Thus, the correct option is B.