Personally I call any material that falls through a 1/8" screen a "fine", so a bucket of these would be, to me, called "pine fines".

What to do with them? Well, as you mentioned, they can be used in 5-1-1 mix in place of peat. Based on my limited experience fine pine material and peat are very similar. Both are acidic, both retain water, etc. So when I make 5-1-1, I generally do not add any peat at all, I simply use some of the very fine (1/8" and smaller) pine material.

I have read where some people use fine pine material for seed starting as well.

TYG

Yes, the fine pine dust can be used the peat is used.
I have used it to increase moisture retention in mixes for ferns. It works wonderfully.

Josh

I vary the amount of peat I add based on the amount of dust in the pine bark fines. I very rarely sift the pine fines, unless I get a real dusty bag.

I have used it mixed 25% pine shake, 75% turface shake for rooting cuttings. It worked OK.

I use bark *dust* with other fines (DE, peat, compost) for my seed starting mixes.

Zeuspaul

I dump excess in perennial beds.

when I have it, I use it 1:1 with peat for seed starting. Or dump it in the compost. You could also use it as soil amendment for sodded areas.

Its been working fine so far for seed starting. I've been using that or a 1:1 ratio of peat to some bagged soil amendment (basically really fine and well composted pine/fir bark) that turned out to be way too small to use for regular growing medium.

Mix in some perlite and use it as seed startting mix. Most seed starting soils are mostly peat moss. I think pine bark fine/dust is better than peat based starting mix. That is exactly what I use.

I'm fortunate enough to be able to buy prescreened fir bark for the gritty mix, and the pine bark I buy for the 5:1:1 mix has never been so fine I need to screen some of the dust out - IOW, I wouldn't find need to screen the bark for the 5:1:1 mix unless it was because it was too coarse. I usually find, with the bark I get, that I need a little peat for moisture retention, but if I had PBFs on hand from screenings, I'd use that up instead of the peat.

If I had a LOT of fine screenings, I'd use them to topdress anything I have growing - the gardens/ beds/ lawn ....

I like coarse seed starting mixes because I don't want to start seeds in a soil I wouldn't grow in, and roots simply grow much better in a well-aerated medium. I usually sprinkle seeds on top of either the 5:1:1 or gritty mix and keep the medium moist. Sometimes, I'll cover seeds in the 5:1:1 with a little peat, or seeds in the gritty mix with Turface screenings. Frequently, I've used an over-turned pot on top of the seeds to lock moisture in but still allow some air circulation via the drain hole.

Al