What does an equal sign mean in chemical equations?
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. [...]
  
  $\ce{H + Br2 -&gt; HBr + Br} \quad$ 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
  $\ce{H2 + Br2 -&gt; 2 HBr}\quad$ net forward reaction
  $\ce{H2 + Br2 &lt;--&gt; 2 HBr}\quad$ reaction, both directions
  $\ce{H2 + Br2 &lt;=&gt; 2 HBr}\quad$ equilibrium
  
  The two-sided arrow $\ce{&lt;-&gt;}$ 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
    $\quad\quad\quad\quad\quad\quad\ce{A -&gt; B + C}$
    $\quad\quad\quad\quad\ce{B + C -&gt; A}$
    It is in this particular case conventional to write these in one line
    $\quad\quad\quad\quad\quad\quad\ce{A &lt;--&gt; B + C}$
  
  
  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
  $\quad\quad\ce{A &lt;=&gt; B + C}$
  $\quad\quad\ce{A =  B + C}$
  
  When one combines a composite mechanism to obtain a net reaction, one should not use the simple arrow in the resulting equation. 
  $\quad$Example 
  $$\begin{align}
\ce{A &amp;\to B + C}\quad\text{ unimolecular elementary reaction} \\
\ce{B + C &amp;\to D +E}\quad\text{ bimolecular elementary reaction} \\
\hline
\ce{A &amp;= D + E}\quad\text{net reaction (no elementary reaction,no molecularity)} 
\end{align}$$