Strange, I thought that one was as close as the regs came to laymans terms.
Use the factor when you don't have allowables based on a test program (and subsequent statistical reduction) or derived from industry standard methods and allowables (such as rivets in sheet metal per MMPDS).
Its basically a requirement for an "extra" ~13% of margin for non-standard or untested installations.
the intention is to prove extra strength for critical parts, like single load paths. i'd say it's application is pretty much hit and miss ... i've seen it applied to standard hardware (counter-point to the previous post), to machined brkts/fttgs, and not ! (sometimes it might be removed from the calc is it creates a -ve MS)
According to Peery, D.J., Aircraft Structures, McGraw - Hill, 1959:
"Many uncertainties exist concerning the stress distribution in fittings. Manufacturing tolerances are such that bolts never fit the holes perfectly, and small variations in dimentions may affect the stress distribution. An additional margin of safety of 15 percent for military airplanes and 20 percent for civil airplanes is used in the design of fittings".
I've used it quire a bit in the design of cargo handling components. They are designed for ultimate ground and flight loads and the extra .15 gives a little more cushion especially when you get into plastic bending.
Bear in mind that per the FAR 25.625 the 1.15 factor applies to all checks (net section, etc.) in the fitting and is not just limited to the joint calcs. Also, joints that are not associated with a fitting (large lap splices, etc.) do not fall under the realm of this FAR. Some DER's; however, do want to see this factor applied to all joints. Also, most OEM's require a minimum +15% margin of safety for fastener shear critical joints. It is not uniform across the industry whether this +15% margin is required on top of a 1.15 fitting factor. In my experience you can wave the +15% min shear margin requirement if a fitting factor is required.
To expand on my previous post- if you have a test program, there should already be some additional factor of safety built in via the statistical reduction (A-Basis confidence level for example). If you have a riveted sheet you have a combination of test allowables (rivet) and a well understood joint behavior in the sheet; it is highly unlikely that a rivet joint designed to be critical in bearing will fail at or before the design load (assuming good practices are followed). So in these cases, the allowable load is not the load at which you would predict the failure to occur at, rather it is the load at which you would predict - with high statistical confidence - that there would NOT be a failure.
Now compare that to the case of a machined or cast "fitting", where you are determining the allowable load via a stress anlysis based on material properties and joint geometry. This allowable load DOES represent the load at which you would predict failure of the part. In this case, there is no built in safety factor. (I wouldn't consider material properties as having a built in factor of safety for stress analysis since they represent minimums for material cert - though if I were to place a wager on a failure load I might). So in these types of cases, I would apply the fitting factor.
I would apply the same general principals to most joints, especially critical ones, whether or not they meet a given definition of "fitting".
