Pretty much the same as in a Proficiency Test. For example, in an Oscilloscope Proficiency Test, there is a dedicated Oscilloscope (such as a Tektronix TDS210 - example). That particular instrument is well characterized, and sent to each participant lab for them to make measurements which are compared against the reference lab and other labs that are participating. So by traveling, it is referring to sending that specific standard around to the participating labs.