Here is also a good summary concerning the TCAS mode control :
http://www.757.org.uk/767/systems/sys2.html
TCAS is normally operated in the TA/RA (TA and RA) mode. Sometimes it may be necessary to operate in the TA Only mode to prevent undesired RAs.
The TA Only mode is used
during engine out operation to prevent RAs when inadequate thrust is available to follow the RA commands. Also, it may be used
when intentionally operating near other aircraft that may cause RAs, such as during parallel approaches and VFR operations.