Per section 296-104-010 of the Washington State Administrative Code for Boilers:
"Low pressure boiler" shall mean a steam boiler operating at a pressure not exceeding 15 psig or a boiler in which water is heated and intended for operation at pressures not exceeding 160 psig or temperatures not exceeding 250 degrees F by the direct application of energy from the combustion of fuels or from electricity, solar or nuclear energy. Low pressure boilers open to atmosphere and vacuum boilers are excluded.
Note: the second part of the above definition is somewhat misleading in that they are describing a water heater as a boiler. In my opinion, it is inappropriate to call a water heater a boiler if you are not actually boiling the water. Nonetheless, thats the definition. Thus the answer to your question is that a low pressure boiler is one which operates at 15 psig or less.
From an engineering aspect, a boiler pressure of 15 psig correlates to a saturated steam temperature of 250 degrees F. A boiler pressure of 100 psig correlates to a saturated steam temperature of 338 degrees F, which is significantly more energetic, and is classified as a power boiler by Washington State. Also, a 1200 psig oil-fired naval boiler correlates to a saturated steam temperature of 568 degrees F, which is 100 degrees hotter than needed to ignite wood. Note that if you add a superheat stage to any of the above boilers, very high superheat steam temperatures are achievable. Regardless if you are running superheat or not, you must control the temperature of the pressure vessel shell, as non-boiler rated carbon steels become very weak at elevated temperatures, and significantly weak at temperatures above 650 degrees F.
Helpful tip: like mild steel, Grade-5 and Grade-8 steel fasteners also lose strength at elevated temperatures. ASTM-A193 steel is used to construct bolts/studs for high temperature applications. ASTM-A194 steel is used to construct nuts for high temperature applications. Like boiler steel, these "high-temp" fasteners are alloyed to retain appreciable strength well up to 1000 degrees F. Note that ASTM-A193 and ASTM-A194 fasteners are readily available through Fastenal, and other on-line supply stores. Theyre not too expensive either. For steam joints, exhaust joints, or any other high-temp joint requiring high strength, these are the fasteners to use.
With regard to the second part of your question - unless the home hobbyist is looking to maximize boiler thermodynamic efficiency (which is unlikely) the safest approach would be to run the lowest possible boiler pressure required, while still obtaining satisfactory model engine operation. What that pressure normally is, I dont know. . . .