The IUPAC Green Book (Quantities,
Units and Symbols in Physical Chemistry, Third Edition, 2007) reads:
2.10.1 Other symbols and conventions in chemistry
[...]
(iv) Equations for chemical reactions
(a) On a microscopic level the reaction equation represents an
elementary reaction [...] A single arrow is used to connect reactants
and products in an elementary reaction. An equal sign is used for the
"net" reaction, the result of a set of elementary reactions. [...]
$\ce{H + Br2 -> HBr + Br} \quad$ one elementary step in HBr formation
$\ce{H2 + Br2 = 2 HBr} \quad$ the sum of several such elementary steps
(b) On a macroscopic level, different
symbols are used connecting the reactants and products in the reaction
equation, with the following meanings:
$\ce{H2 + Br2 = 2 HBr}\quad$ stoichiometric equation
$\ce{H2 + Br2 -> 2 HBr}\quad$ net forward reaction
$\ce{H2 + Br2 <--> 2 HBr}\quad$ reaction, both directions
$\ce{H2 + Br2 <=> 2 HBr}\quad$ equilibrium
The two-sided arrow $\ce{<->}$ should not be used
for reactions to avoid confusion with resonance structures [...]
2.12.1 Other symbols, terms, and conventions used in chemical kinetics
[...]
(ii) Composite mechanisms
A reaction that involves more than one elementary reaction is said to occur by a composite mechanism. The terms complex mechanism, indirect mechanism, and stepwise mechanism are also commonly used. Special types of mechanisms include chain-reaction mechanisms, catalytic reaction mechanisms, etc.
Examples
A simple mechanism is composed of forward and reverse reactions
$\quad\quad\quad\quad\quad\quad\ce{A -> B + C}$
$\quad\quad\quad\quad\ce{B + C -> A}$
It is in this particular case conventional to write these in one line
$\quad\quad\quad\quad\quad\quad\ce{A <--> B + C}$
However, it is useful in kinetics to distinguish this from a net reaction, which is written either with two one-sided arrows or an "equal" sign
$\quad\quad\ce{A <=> B + C}$
$\quad\quad\ce{A = B + C}$
When one combines a composite mechanism to obtain a net reaction, one should not use the simple arrow in the resulting equation.
$\quad$Example
$$\begin{align}
\ce{A &\to B + C}\quad\text{ unimolecular elementary reaction} \\
\ce{B + C &\to D +E}\quad\text{ bimolecular elementary reaction} \\
\hline
\ce{A &= D + E}\quad\text{net reaction (no elementary reaction,no molecularity)}
\end{align}$$