Brian, you are doing fine. Keep asking questions, too.
I don't understand what the difference between TWN and SWN is
TWN is what it indicates - the absolute Total Water Needed as an ingredient for the recipe. SWN can be different due to the fact that one might be holding water back in Section W, BUT if Section W is blank, SWN will be greater than TWN because water expands as it is heated. I am not sure how to answer your question if you are steeping grains in some of the TWN outside of the mash in the kettle. I don't steep grains separately.
I expect that Mash Volume is water plus malt, and thus VIB is liquor remaining once the bag of malt has been removed.
You are correct - the mash volume is how full the kettle will be for the time of mashing. VIB is to be measured at the start of the boil. The bag was pulled well before this at a lower temperature. This can be a little fuzzy because foaming sometimes occurs at the same time as boiling starts and it makes measuring difficult (especially if you are brewing solo). Move the spreadsheet on your screen to see Sections U and V over on the right. Section U is the height of the liquid in your
kettle according to the dimensions you entered in Section B. Section V is the headspace, or the empty part of your
kettle from the top of the liquid to the rim. I prefer to measure without sticking anything other than a thermometer into the liquid, so a metal tape measure works well to measure the headspace*. VIB can also vary according to the spent grain holding different amounts of liquid depending on the recipe or the brew day, or squeezing.
Is VFO higher than VAW because of crud left in the kettle? If so, why is there a further loss (KFL) separate from the VFO VAW loss?
VFO / VAW difference is really due to the volume difference related to temperature. The difference between VAW and VIF is KFL. The KFL in my experience is variable because the amount of trub and how much of it gets transferred from Kettle to Fermenter is both recipe dependent and related to my transfer technique on that day. I am not very picky about keeping all the trub out of my primary fermenter. This also entails having accurate marks on fermenters, or measuring headspace prior to fastening the lid down.
Finally, how do I measure the volumes VIB and VAW accurately? My kettle does not have volume marking on it.
I'll have to look for a link on how to scribe long-lasting markings on a kettle wall, it was a long time ago that I saw it. I chose to not do that because of resulting scratches or bumps in my kettle wall. However, you can mark the outside with a sharpie. It is not super accurate, but it may help. Sections U and V again (see above).
Keep asking questions!
*Technically, the headspace measured also depends on the temperature of the liquid and the expansion associated with it when hot. How precise does one need to be? TWN @ STP?