Refer to figure.
ICAO Annex 11 - Air Traffic Services
CHAPTER 1. DEFINITIONS

Automatic terminal information service (ATIS). The automatic provision of current, routine information to arriving and departing aircraft throughout 24 hours or a specified portion thereof:

Data link-automatic terminal information service (D-ATIS). The provision of ATIS via data link.

Voice-automatic terminal information service (Voice-ATIS). The provision of ATIS by means of continuous and repetitive voice broadcasts.

Voice ATIS (Automated Terminal Information Service) is a continuous broadcast of recorded information in busier airports. The primary purpose of ATIS is to reduce the controller workload and provide pilots with essential information about the airport and its immediate surroundings. In order to receive the Voice-ATIS, the pilot has to tune a given VHF frequency.

The ATIS has a letter identifier (A, B, C, D, ... Z). The pilot uses it to indicate reception of the appropriate ATIS to the ATC. ATIS information is usually updated every 30 minutes, and the letter increases after the update (i.e. ATIS information A is updated to ATIS information B).