When it is done/advised ?
Holter monitoring is essentially 'mobile ECG', meaning the ECG trace is recorded while patient is continuing with his routine, rather than being confined to bed.

This device to record ECG is advised when a patient is suspected to have periodic ECG changes during the course of his routine activity, and may not show any changes when reported to the doctor. The test is advised most frequently when a patient may have unexplained palpitations, fainting or other rhythm disorders.