Extension cords have mysterious letters and numbers printed on the jacket to show the uses for which they are appropriate. Things like amperage and voltage capacity and appropriateness for use in extreme conditions are what those codes identify.
The code is contained in the National Electrical Code and understanding it makes buying extension cords easier.
Here is what some of those codes refer to:
S Hard Service Flexible Cord
SJ Junior Hard Service Flexible Cord
E Thermoplastic Elastomer Insulation
T Thermoplastic Insulation
O Jacket is Oil Resistant
OO Jacket & Conductors are Oil Resistant
R Thermoplastic Rubber Insulation
X Cross-linked Synthetic Polymer Insulation
HH High Temperature
W Moisture Resistant
N Nylon Jacket
Our line of white extension cords from Century Wire have SJTW coded jackets, meaning that have a hard service flexible cable with thermoplastic insulation and they are moisture resistant and appropriate for outdoor use.