@Pratheek Punchathody's answer isn't very clear. The white boxes are terminals of a Specialized Power Systems (SPS) block. The grey ones are those of a Simscape (SSC) block. These can be interfaced using interface blocks that MATLAB provides.
SPS has some special components (like the Controlled Voltage Source block, and an Ideal DC source)
SSC is what we commonly use when building circuits on Simulink (Voltage source, H-bridge, etc)