https://www.math.utah.edu/vigre/annual-report/ncolorability.pdf
The above pdf about Reidemeister moves contains this symbol
$$\Leftrightarrow$$
It says "The expression for the crossing of a single twist is $a_{1} + a_{2} \Leftrightarrow 2a_{1}$" where $\Leftrightarrow$ seems to be modular congruency.(pg 3, Reidemeister Case 1)
It also says "There are now four strands $a_{1},a_{2},a_{3},a_{4}$ and two crossings which most must satisfy $a_{1} + a_{3} \Leftrightarrow2a_{2} \equiv 0$." Here , $\Leftrightarrow$ seems to be about subtraction or something.
Could someone please explain what this symbol means? Wikipedia claims it has to do with logical equivalence, though is sometimes used incorrectly for "iff". But neither of these makes sense to me in this context.