From the IRS:
The name control consists of up to four alpha and/or numeric characters. It can be determined from the information specified on the first name line. Generally, the name control is derived from the first four characters of the partnership name. An ampersand (&) and hyphen (-) are the only special characters allowed in the name control. The name control can have less, but no more than four characters. Blanks may be present only at the end of the name control.
See Using the correct name control in e-filing partnership tax