bip means bitcoin improvement proposal.  I don't think there is a formal convention behind the numbering scheme, just that its increased to a larger number  when a new one is created. 

Take a step back - there is a public bitcoin developers mailing list.  Anyone can sign up.  The community encourages everyone who wishes to work on or contribute to bitcoin core, to use this mailing list and subscribe to it.  The goal is that a developer will come up with a great idea then he will discuss it with other users on the mailing list to gain feedback about it.  If the feedback is overtly positive or at least agreed upon but given some constructive feedback, THEN, the developer is encouraged to formalize his proposal with a synopsis that includes sample code for implementation, test results of such implementation as well as the benefits of implementing such change.  This 'proposal' is a new BIP!  it gets drafted and submitted to github, and one of the bitcoin project committers places it into the BIP directory, which is hosted on github. From there, I'm not sure how the core developers choose which ones to implement and how they go about moving forward.  I think things all started breaking down once we got to bip101 - its a formalized proposal that is essentially XT (8mb blocks with doubling each year until 2032 when the blocksize is expected to be as large as up to 8GB), but it seems that the core developers have a fundamental disagreement as to the implementation of it or which one to implement (bip 102 is Jeff Garziks conservative 2mb increase), (bip103 is Pieter's De Wiulle's proposal to do gradual blocksize increases on a much longer timescale.

Thanks Frank, that's what I was looking for.  I wasn't sure if it was a bitcoin only thing and how they came up with the numbers.