You may recall I mentioned "Male" & "Female" a few times while describing some of these various connectors at the article The Most Common RC Lipo Battery Connectors You Should Know. And in the next picture I show one sex of connector as the LiPo/Source, and the other as the ESC/Device.

xt30 plug

Why is this important?

Basically, any connector that has power at the pins (ie. the one on the battery side or BEC side), must be the one out of the two that is the least likely to short out! This is very important for safety reasons and I have often seen people new to the hobby solder the wrong sex on the wrong end. This will still work of course, but you risk shorting out the LiPo pack if you solder the easy to short connector on the battery side instead of the device side.

So which is the least likely to short?

A little common sense is all that's required to figure it out. Simply look at each plug end and figure out which one is easier to accidentally contact both pins at the same time (shorting them out) with a metal or electrically conductive object, such as the tip of a screw driver that might be floating around in your RC field box along with several battery packs.

No question you could short the other connector out with something like a bare wire bent in a "U" shape, or pair of needle nose pliers / tweezers inserted into each insulated pin, but it's much less likely to occur.  

connectors

This is why most people in the hobby don't care for the non-specific plugs that can be used for both source and device. Even with them however, there is a safety rule that must be followed. The exposed pin on the battery side must always be the negative "-" and the encased pin must be positive "+".

Same holds true for exposed bullet pins. The female bullet that is insulated with heat shrink will always be on the power side and the exposed male pin will be on the device side.