What is the difference between $\Longleftrightarrow$ and $\Leftrightarrow$?

Question

this post gives some explanation about the definition and operation of sets.

the paragraph above Axiom 1b uses this symbol $\Leftrightarrow$ to indicate that p and q ­are equivalent.

the paragraph below Axiom 1b uses this symbol $\Longleftrightarrow$ to give the Definition of "Union of two sets"

so, is the difference between $\Longleftrightarrow$ and $\Leftrightarrow$ is as follow?

$\Leftrightarrow$ represents equivalent

$\Longleftrightarrow$ represents definition