Difference between memo and circular with references?

Memo is a kind of letter that contains specific information which applicable to someone directly and a circular is applicable to all working in an organisation.

A memo needs a reply whereas a circular may not need any reply cause it contains law in force or any kind of additional information of rule/law.

A memo may be for good as well as bad for a person it may contain good news also and bad news also for an official but a circular is free from these.

A memo can be approached by the competent authority of an organisation from a particular sector/region/branch or local head offices but a circular always approached by the head of department of leaded by rule or law.