title: What is the difference between bind variables and substitution variables(which I input using &&)?
body: <p>What is the difference between these two variable declarations?</p>
  
  <p>1: <code>num number:='&amp;&amp;num';</code><br>
  2: <code>variable num1 number;</code></p>
  
  <p>Since in both cases I can reference <code>num</code> by using <code>&amp;num</code> or <code>&amp;&amp;num</code> in other files also, 
  and in the case of bind variables <code>:num1</code>. </p>
  
  <p>Moreover I have one more confusion: whether any of the below statements differ somehow, are they both valid and do they mean the same thing?</p>
  
  <p>1: <code>variable num1 number;</code><br>
  2: <code>var num1 number;</code></p>