The following is taken from the CFMU manual:
EUROCONTROL BASIC CFMU HANDBOOK
CFMU ATFCM USERS MANUAL
4-6
4.3.3.4. Ready Procedure
For flights having already received their slot and being in a situation to depart before their CTOT (doors closed and ready to depart), the AO may ask local ATC to send a Ready (REA) message. A MINLINEUP time may also be included in the REA, to indicate the minimum time needed for that flight to get from its position to take-off. REA may be sent between EOBT minus 30 minutes and the CTOT of the flight. When the REA is filed before the EOBT, the flight is considered as having a new EOBT at this filing time and the MINLINEUP as a revised taxitime. To keep track of the difference between the
filed off block time and the effective one in ETFMS all subsequent ATFCM messages include the field(s) IOBT and possibly IOBD (IOBT = latest EOBT filed before the REA was sent). If an improvement is possible, it will be provided with an SRM.
At my airfield REA messages are sent with a minimum line up of 1, 5, 10, 15 or 20 minutes as standard, but any number can be selected via a secondary system.
SRM = Slot Revision Message (i.e. a better slot - hopefully!
).
A REA message may only be sent by ATC; Airline Operators (AOs) may send RFI (Ready For Improvement) which works slightly differently. (I don't believe a RFI message can alter the taxitime).
You can find more here:
http://www.cfmu.eurocontrol.int