There are quite a few different power cord designations (SJO, SJOO, SJOW, etc). The reference chart below lists and explains the variations.
Legend:
S = Service Grade (also means extra hard service when not followed by J, V, or P)
J = Hard Service
V = Vacuum cleaner cord (also light duty cable)
P = Parallel cord (also known as zip cord) – Always light duty
E = Thermoplastic Elastomer (UL/NEC designation ONLY)
O = Oil Resistant*
T = Thermoplastic
W = Outdoor-includes sunlight resistant jacket and wet location rated conductors (formerly "W-A")
H = Heater cable
VW-1 = Flame retardant
FT2 = Flame retardant
* When only one "O" appears in a classification (i.e. SJEOW), only the outer jacket material is oil resistant. If two "O's" are in the classification (i.e. SEOOW), the insulation covering the conductors and the outer jacket insulation are all oil resistant
Definitions of Cable Types:
SVT — Thermoplastic insulated vacuum cleaner cord, with or without 3rd conductor for grounding purposes; 300V. (PVC)
SJT — Junior hard service, thermoplastic insulated conductors and jacket. 300V. (PVC)
SJTW — Same as SJT except outdoor rated. (PVC)
SJTO — Same as SJT but oil resistant outer jacket. (PVC)
SJTOW — Same as SJTO except outdoor rated. (PVC)
ST — Hard service cord with all thermoplastic construction, 600V. (PVC)
STW — Same as ST except outdoor rated. (PVC)
STO — Same as ST but with oil resistant outer jacket. (PVC)
STOW — Same as STO except outdoor rated. (PVC)
SPT-1 — Parallel jacketed thermoplastic cable, 300V. With or without third conductor for grounding. (PVC)
SPT-2 — Same as SPT-1 but heavier construction. (PVC)
SPT-3 — Same as SPT-2 but heavier construction. (PVC)
SJE — Hard service cord, thermoplastic elastomer insulated and jacketed, 300V. (TPE)
SJEW — Same as SJE except outdoor rated. (TPE)
SJEO — Same as SJE but with oil resistant jacket. (TPE)
SJEOW — Same as SJE except outdoor rated. (TPE)
SJEOO — Same as SJE but conductor insulation and jacket BOTH oil resistant. (TPE)
SJEOOW — Same as SJEOO except outdoor rated. (TPE)
SE — Extra hard service cord, thermoplastic elastomer conductor insulation and jacket, 600V. (TPE)
SEW — Same as SE except outdoor rated. (TPE)
SEO — Same as SE but with oil resistant jacket. (TPE)
SEOW — Same as SEO except outdoor rated. (TPE)
SEOO — Same as SE but with oil resistant conductor insulation and jacket. (TPE)
SEOOW — Same as SEOO except outdoor rated. (TPE)
S — Extra hard service cord, thermoset-insulated conductors and thermoset jacket, 600V.
SO — Same as S but with oil resistant jacket. (thermoset)
SOW — Same as S except outdoor rated. (thermoset)
SOO — Same as S but both conductor insulation and jacket are oil resistant. (thermoset)
SOOW — Same as SOO except outdoor rated. (thermoset)
SJ — Hard service cord, thermoset insulated conductors and thermoset jacket, 300V. (thermoset)
SJO — Same as SJ but with oil resistant jacket. (thermoset)
SJOW — Same as SJO except outdoor rated. (thermoset)
SJOO — Same as SJ but both conductor insulation and jacket are oil resistant. (thermoset)
SJOOW — Same as SJOO except outdoor rated. (thermoset)