In the BITalino Board version the sensors are already connected to the main board. You just need to connect the electrode cables to work with the sensors. No other sensors can be connected to this product version.
In the BITalino Plugged version, the BITalino Sensors (barebone version with UC-E6 connectors) can be connected to the BITalino Core (battery is not included) through a Sensor Cable that connects the sensor to the main board and then one 2- or 3-Electrode Lead Cable that connects the sensor to the body. This version is compatible with all sensors from the BITalino portfolio (to know more about sensors compatibility visit this article). To know more about how BITalino and Sensors can be connected you can check the BITalino User Manual (Section 2.2).