Notation of m(B_n) & m(A): Explained

[Zorba]

In a book I am reading I see the following:

$$m(B_n) \uparrow m(A)\quad \textrm{if}\, B_1 \subset B_2 \subset \dots\, \textrm{and}\, \bigcup_{n=1}^{\infty} B_n = A$$

What does the up arrow signify? m(A) denotes the lebesgue measure of A if that helps.

 

[Mark44]

My guess is that it means "approaches from below." IOW m(B_n) < m(A) for all n, but as n increases the difference between m(B_n) and m(A) becomes smaller.