A USB A plug can be mounted onto the PCB allowing the prototype to be plugged directly into a USB power outlet (PC, Wall brick, battery etc.).
The A and B inputs to the Electron have pull-up 1206 pads (PCB top layer) to 3v3 and pull-down 1206 pads (PCB bottom layer) to GND, this makes it easy to create a potential divider on the analog inputs for resistive sensors such as LDRs, thermistors, variable resistors etc.
8x 1206 pads at the end of the PCB (top and bottom) for LEDs, resistors, capacitors, input protection (emc filters, resistors etc).
10 way connector with 3v3 and GND connected and 8 connections to one side of the 1206 pads.
4 1206 pads top and bottom for SMD capacitors, resistors etc by the main prototyping area to make room for more interesting components.
PCB bottom layer underneath the electron has pads for a SMD transistor or small FET with support resistors (base/gate protect and pull down).
3x mounting holes for M3 machine screws or A2 self tappers.