Threshold?

My current definition of the threshold:
A sharp point of difference spatially between two spaces - that can either be different, opposing or the same - which contains a certain mechanised access or ritual of activation allowing it to be selectively-permeable (i.e. access designed for certain people or designed to exclude certain people) but requiring a physical 'crossing' of the connection to result in a condition, transition or phemenon to occur or simply to 'enter' the second space. The threshold in architectural terms could be significantly 'different' for example the gate into the Taj Mahal, or simply a line dividing two spaces.

Few things that do puzzle me are the reasons why these connections are required i.e. when does a room require a door, what functional roles do they play, are there any alternatives to connecting things, what kind of passage is required... From research:

Doors - is used to control the physical atmosphere within a space by enclosing it; that is, it is a process of screening what is inside the door but also giving the impression of an entrance to it. It allows the differentiation from in to out, controls the conditions (e.g. when the door is opened, light and air enter).

Cells - selectively permeable to allow only certain things in, made from protein, segregation of biological interior to exterior things. It keeps things inside...

Perhaps, the connection is a mutual point between inside and outside which requires some sort of segregation, such that the purpose is to allow limited access... The threshold influences the two spaces by imparting an impression of entrance/exit/passage... E.g. a cell membrane exerts influence onto the inside by allowing only a few things in and a few things out... That boundary kind of influences it, allows us to identify whats in and whats out? A factor of movement between two spaces? A factor of occupation (i.e. it should be used)? 


The doors locked in archi building can be considered as walls because they dont allow passage through it, though, have the potential to.