You can precede a trigger with printer-specific control characters. The most common reason to do so is to ensure the printer receives the job you send as a new job.
A printer expects each job that it handles to end with a special character that tells the printer it has reached the end of the input data. Until the printer receives this special character, it continues to process all input it receives as part of that job. If there is no input, the printer waits for a defined period of time, then times out and proceeds to the next job. If a new job arrives during the period of time the printer is waiting for input, the printer does not recognize it as a new job; rather it processes it as input for the current job.
It is thus common practice to include an end of job character at the beginning of the trigger to ensure that the printer recognizes your job as a new print job. For certain printers, <CTRL D> or ASCII 04 is a valid end of job character, while more recent printers require a Printer Job Language (PJL) sequence such as <ESC>%-12345X<CR><LF>.
As an example, the following trigger includes <CTRL D> as an end of job character:
<CRTL-D>%!PS-Adobe <CR><LF>
run INVOICE <CR><LF>