T.120 is an ITU-T recommendation that describes a series of communication and application protocols and services that provide support for real-time, multipoint data communications. It is used by products such as Cisco

Cisco

Cisco may refer to:Companies:*Cisco Systems, a computer networking company* Certis CISCO, corporatised entity of the former Commercial and Industrial Security Corporation in Singapore...

 WebEx

WebEx

WebEx Communications Inc. is a Cisco company that provides on-demand collaboration, online meeting, web conferencing and videoconferencing applications...

's MeetingCenter, Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 NetMeeting, Nortel

Nortel

Nortel Networks Corporation, formerly known as Northern Telecom Limited and sometimes known simply as Nortel, was a multinational telecommunications equipment manufacturer headquartered in Mississauga, Ontario, Canada...

 CS 2100

Nortel Communication Server 2100

The Avaya Communication Server 2100 or CS-2100 in computer networking, telecomunications and IP telephony is a carrier grade, IP soft switch or VoIP server solution for large campus and geographically dispersed large enterprises...

 and Lotus Sametime

Lotus Sametime

IBM Sametime is a client–server application and middleware platform that provides real-time, unified communications and collaboration for enterprises. Those capabilities include presence information, enterprise instant messaging, web conferencing, community collaboration, and telephony capabilities...

to support application sharing, real-time text conferencing and other functions.

The set of "T.120" recommendations includes:

• T.120 - Data protocols for multimedia conferencing
• T.Imp120 - Revised Implementor's Guide for the ITU-T T.120 Recommendation series
• T.121 - Generic application template
• T.122 - Multipoint communication service - Service definition
• T.123 - Network-specific data protocol stacks for multimedia conferencing
• T.124 - Generic Conference Control
• T.125 - Multipoint communication service protocol specification
• T.126 - Multipoint still image and annotation protocol
• T.127 - Multipoint binary file transfer protocol
• T.128 - Multipoint application sharing (was known as T.share while in draft stage)
• T.134 - Text chat application entity
• T.135 - User-to-reservation system transactions within T.120 conferences
• T.136 - Remote device control application protocol
• T.137 - Virtual meeting room management - services and protocol

The PCMag encyclopedia of computer terms lists the following additional recommendations:

• T.130 - Realtime Architecture - Interaction between T.120 and H.320.
• T.131 - Network Specific Mappings - Transport of realtime data used with T.120 over LANs.
• T.132 - Realtime Link Management - Creation and routing of realtime data streams.
• T.133 - Audio Visual Control Services - Controls for realtime data streams.
• T.RES - Reservation Services - Interaction between devices and reservation systems
• T.Share - this is now known as T.128
• T.TUD - User Reservation - Transporting user-defined data

These are not listed on the official ITU-T website.
It is possible that these recommendations have been withdrawn.

T.RES and T.TUD are possibly recommendations that did not make it out of draft stage; the ITU practice is to use the series letter + an alphanumeric code for the recommendation while it is in draft, and a number after publication.