What's the purpose of "FLAGS" in tensorflow

I am studying the mnist example in tensorflow.

I am confused with the module FLAGS

# Basic model parameters as external flags.
FLAGS = None

In the "run_training" funcition :

def run_training():
"""Train MNIST for a number of steps."""
# Tell TensorFlow that the model will be built into the default Graph.
with tf.Graph().as_default():
# Input images and labels.
images, labels = inputs(train=True, batch_size=FLAGS.batch_size,
                        num_epochs=FLAGS.num_epochs)

What's the purpose of using "FLAGS.batch_size" and "FLAGS.num_epochs" here? Can I just replace it with a constant number like 128?

I found a similar answer in this site but I still can't understand.