SCHED was originally written as the scheduling program for global VLBI observations using the Mark II recording system. That was a system capable of recording 4 Mbps on video tape recorders. It was in use in some parts of the world much longer than expected (at least to 2003), but is now gone as far as we know. SCHED version 9.4 and later does not support Mark II.
The wide band recording system in use for VLBI for many years was the Mark III system. SCHED was never a general purpose scheduling program for Mark III observations. Programs SKED and PC-SCHED served that role. However, SCHED was capable of scheduling Mark III observations on systems that used the VLBA control files (VLBA, VLA, and Green Bank). In fact, the output of SKED and PC-SCHED was normally processed through SCHED to produce the telescope control files for these antennas for all except geodetic projects. Mark III is also obsolete, although one Mark III schedule is still in the SCHED test suite. I am not aware of any Mark III systems remaining in use.
The Mark III system was replaced by several other systems with greater capabilities. These include the VLBA, VLBA4, Mark IV, S2, and K4 tape systems. There are correlators associated with each system, and there is a considerable amount of cross compatibility, either directly or through the use of translation machines. The Mark III, VLBA, VLBA4, and Mark IV systems all used the same tape transport, although with different electronics. In their native modes, they use different data formats, but the VLBA and Mark IV systems are capable of reading and writing Mark III data. More importantly, the VLBA and Mark IV systems have a wide range of compatible recording modes that can be correlated together on the VLBA, JIVE, and other correlators.
By 2007, the tape systems were replaced by disk based recording systems nearly everywhere. Most widespread is the Mark5A system developed by Haystack Observatory and Conduant Corporation. The Mark5A system is a plug replacement for the old tape drives and still has the concept of tracks. That is the most common system as of early 2008. The Mark5B system uses the VSI standard interface to record the data channels without all the formatting baggage left over from the tape systems. The Mark5A+ system allows Mark5B recordings to be played on Mark5A playback units, but a minor glitch that nobody is in a position to fix is preventing its use on the VLBA. The Mark5B+ system uses a faster interface and can handle up to 2 Gbps. The Mark5C system is being deployed in 2011 and records up to 4 Gbps. The Mark5C- system is a scheme for allowing lesser Mark5 hardware to pretend to be a Mark5C system for system development. Confused yet? The VLBA is using (early 2011) the Mark5A system and is beginning to test the Mark5C system, in an initial 2 Gbps mode, at the telescopes. Mark5A will likely be supplanted by Mark5C on the VLBA during 2011.
There are other recording systems in use. The Japanese have a K5 system. The Australians are using a variant on the PCEVN system. Also a variety of groups are testing real time VLBI over the fiber networks. This can involve real time correlation, or recording at sites remote from the telescope.