A catheter is a medical device that is used to drain urine and aid in urination.20ml and 30ml refer to the size of the capacity of the catheter, which is the volume of the lumen inside the catheter.A 20ml catheter has a capacity of 20ml, while a 30ml catheter has a capacity of 30ml. The choice of volume is usually based on the patient's specific situation and needs. Smaller catheters are used in special cases such as children, newborns, or patients who require less drainage. Larger catheters are used for patients who require greater drainage, such as adult patients with high urine output. Choosing the right catheter size ensures effective urine drainage and reduces discomfort or complications.