I was told that abbreviations consist of initialisms (_FBI_), acronyms (_NATO_, _a.k.a._†), and shortened words (_ad_, _bike_).

† _As Pete Kirkham have corrected me, for most people a.k.a. is an initialism._

But what about _i.e._ and _e.g._? At the first glance, they look as initialisms, but typically we pronounce them as "for example" and "that is", not literally, like "eye ee".

So which subclass of abbreviations they belong to? Initialisms or maybe something different, and why? Are there reputable sources to classify them?