I would say the phrase, "more durable," is squarely in the realm of marketing jargon and is of little use in determining whether the pads are of the two general categories of disc brake pads, "resin" and "metal", which DO carry connotations related to rotor wear with them. These terms, while helpful in determining pad and rotor compatibility and for a few overall expectations of performance characteristics, are a little misleading from the reality of what disc pads are. Both classes of pads have resin bound fibers of metal (copper alloy)to a more or less degree.
BO3S & BO5S sit within the "resin" category of pad. The characteristic ratings are identical between the two models, as are the compatible hydraulic calipers. Being "resin" class pads they can be used with all rotors compatible with the calipers. Apparently even official Shimano product documents are not free from a marketing opportunity, as one can see Shimano notes how the BO5S is "40% more durable" by means of an asterisk and footnote.
While I would guess the "more durable" claim here refers to resistance to wear and increased lifespan, and/or resistance to performance fade during usage instances that generate a lot of heat. The fact they are classed as "resin" pads, mean we can then only expect a characteristic of generally lower wear to a rotor. Lower wear compared only to the general characteristic of the "metal" class of pads.
Rotors can have compatibility concerns in regard to the type of pad that can be used with them. Some rotors--generally those at the lower tier of the hierarchy, and lower price points--are designed such that only "resin" pads should be used with them. There is no further pad material limitations (like "use only with 40% more durable" resin pads) given other than they be in the "resin" class.. That, then, is the practical information required to equip disc brakes. Your question is a good one, and I wonder too, about the details surrounding the durability statements. The fact is that both resin models may be used for any given compatible rotor. The inference is that a difference in the degree of wear to that rotor is negligible. Certainly not 40 or any significant percentage more or worse wear to a rotor going from a B03 to a BO5.
Obviously, there is a significant degree of difference in rotor wear between a given "resin" class pad and it's "metal" counterpart. Hence, the "resin only" designation of some rotors. And useful rotor life is diminished when metal pads are used exclusively.
Rotor life typically ends when the thickness reaches 1.5 to 1.55mm, depending on manufacturer. You'll get to that mark sooner using metal pads on a given rotor than you would if it's resin counterpart is used. Any anecdotal evidence of a higher degree of wear between pads of the same class (resin or metal) is something I've not seen discussed. It may or may not exist, but there hasn't been a significance tied to it other than one can expect "resin pad" qualities from both resin models. In terms of wear, that would be less than with a metal pad choice.
