This is 1oz spool with a #66 core of #48 flux. 66 means that 3.3% of the wire is flux by weight. This is recommended for most applications, but versions with less flux are also available.
#48 flux is a RA (rosin-activated) flux designed to help solder quickly “wet” or adhere to a variety of surfaces. Most manufacturers would clean activated flux residues, but most applications (hobby, prototype building, consumer electronics) are likely fine with no cleaning. From the #48 flux spec sheet: “The 48 residues are non-conductive, non-corrosive and do not require removal in most applications. The
flux residues are comparable to a conventional RA.”
The residues dry fairly clear with much less of the amber color of #44 flux.
Comparing K100LD to the more commonly used SAC (tin silver copper): In our testing, we haven’t noticed much difference, except that the joints are shinier with the K100LD. For lead-free solder, however, the shininess of the finished joint is not a good quality indicator–most SAC joints come out duller, and this is perfectly normal.