COVERLET alerts the program that all lines after the next '/' and up the the occurance of 'endcover' and '/' on a line will be text that is to be transfered to the summary and operator schedule files (sch. files). This allows the PI to write a cover letter that will make it into files that the operators at at least some stations will see, hopefully eliminating the need to write a separate cover letter file. The amount of text is arbitrary and the line need only be shorter than 256 characters. Lines over about 80 characters may not look good in the output.
Note that the operators at some telescopes, including the VLBA, do not pay much attention to what is in the files. Cover letters of any type are not very effective. Your observation should be described well by the parameters that actually control telescope operations. Cover letters can be effective for observatories where some operations have to be done by hand.
The egglobal.key example shows the use of COVERLET.