Practical Sources emulate sources we encounter in the real world, they consist of an ideal source connected to an equivalent resistance representing the internal resistance of the source.
They are also unable to provide the exact rated voltage or current to the load due to this internal resistance. There is a voltage/current divider effect. In other words, the output is not constant as for the case of ideal sources and depends on the load.
Practical Voltage Sources
A practical voltage source is an ideal independent voltage source in series with its internal resistance:
A practical voltage source's output will be closer to the rated value for attached loads with a higher resistance so that minimal voltage will be dropped across the internal resistance.
Practical voltage sources with lower internal resistance are able to more effectively provide the rated voltage to the attached load.
Practical Current Sources
A practical current source is an ideal independent current source in parallel with its internal resistance:
A practical current source's output will be closer to the rated value for attached loads with a lower resistance so that most of the current will flow through the load.
Practical current sources with higher internal resistance are able to more effectively provide the rated current to the attached load.