What does this circuit symbol mean? (Circle with horizontal line through it)
It's a fixed current source (DIN 40700 style). With an arrow pointing to it an externally controlled current source. In this case it's a CCCS (Current Controlled Current Source), where the generated current is the controlling current divided by 500, resp. 1000, depending on the device.

Other common current source symbols include:  





Controlled current sources may be indicated by:  

  

or