I hope I don't bother you with so many questions.
I can see in specifications the value of 58-60 RC hardness for SR-77 and INFI. What does it mean those two values? Are they differentially heat treated having 60 at the edge and 58 on the spine and handle? Or it's just the homogenous hardness is not exactly determined and may be anywhere in that area?

Generally refers to the edge. If diff treated, release specs will indicate as much.

So the edge is 60 RC and the rest is 58 RC.

But do you know how wide is the 60RC portion from the edge? How deep from the edge starts the 58RC?
And what happens during the use after I "eat" more from the edge by resharpening? Shall I have only some lose in edge retention when the edge will be at half of the actual blade width?

Perhaps, but not always. 58-60 refers to an HRC range. Treat a tool/cutlery steel to an HRC within that range and you have known properties of toughness and wear resistance as published by the material supplier.

58-60 is a range of hardness for the hardened area which may or may not include the whole blade as Adamlau stated. These are typically general specs that are put out by manufacturers to measure hardness given the appropriate heat treating process. Since they don't test every piece of hardened meal that they produce and since there may be some error in the heating, quenching and tempering processes, a range is used to specify hardness instead of a discrete value. The typical spine on a differentially heat treated blade should not be anywhere near a value of RC58 - to do so otherwise would invalidate the reasoning for differential heat treating in the first place (remove brittleness and restore resiliency to the spine).

So, they are not differentially heat treated.
They are homogenous. That's what I also found out from this link: http://www.bussecollector.com/INFI/
"INFI is the only knife steel ever tested that has achieved such high levels of lateral strength with a homogenous hardness of 58 - 60 Rc"
But I was not sure.

Thanks for clarifying that, guys!

So, 58-60 is the tolerance area where the blade is and it's intended to be homogenous.

Correct, unless specifically indicated otherwise.

Snotpig, I'm almost certainly being overly picky and with no disrespect intended, I think you probably meant variance rather than error. It's just that (at least to me) error implies slipshod QA, whereas variance might be more appropriate for specifying that the hardness will fall between an industry-standard range of values.

Shun and Warren Thomas knives are also laminated along those lines, though WT lines Ti with either CF, or G-10.

The posts above are correct, but to bring a little more technicality to this discussion an R'C' value refers to the Rockwell Hardness value of a certain metal. There are three scales on the Rockwell Hardness scale : A, B, and C. The scales progress from softer to harder A is softer than B which is softer than C. Softer metals are usually measured on the A and B scales but heat treated steel being so hard has to be measured on the C scale. The three scales overlap and that's where the Rockwell measuring system can be quite misleading. The A scale overlaps the B scale and the B overlaps the C. So a high RA (read: Rockwell A value) can mean the same hardness as a low RB and a high RB can mean the same as a low RC. Some people who don't like this tend to prefer the Brinell Hardness scale which is significantly more simple to use. The Brinell hardness (BH) scale is a metric (I think) measurment, so if you see some Euro-made blade advertised with a BH value, it's most likely to mean the blade's hardness. Sorry for being so technical, but maybe it will be useful to someone...