Transistors - What is re model?
The so-called "re-model" is a BJT model which is - according to my opinion - not very close to te physical reality and, hence, not very useful. In contrast, I think it is even confusing because some people (even authors) call this element "inherent emitter resistor" which is simply wrong. 

It is not a resistive element and it does not belong to the emitter.

Instead, it is simply the inverse of the transconductance gm (re=1/gm), which connects the controlling input voltage Vbe with the controlled quantity (collector current Ic): 
gm=d(Ic)/d(Vbe). 

(For my opinion, to understand the real transistor function it does not help at all to use the inverse transconductance re=1/gm within the model and to treat it as a resistive element). 

Comment: The comment above from Sparky256 is a very good example for the confusing use of this symbol. An external emitter resistor Re (causing negative feedback) is an ohmic linear resistor (large letter) whereas a symbol with small letters (re) always indicates a dynamic (differential) resistor.