EO, or Eight Ones, is an 8-bit EBCDIC character code represented as all ones (binary 1111 1111, hexadecimal FF).
Eight Ones, as an EBCDIC control code, is used for synchronisation purposes, such as a time and media filler.
[1] In Advanced Function Presentation code page definition resource headers, setting at least the first two bytes of the field for the eight-byte code page resource name (which is encoded in code page 500) to Eight Ones (0xFF) constitutes a "null name", which is treated as unset.
[2] When translated from the EBCDIC character set to code pages with a C1 control code set, Eight Ones is typically mapped to hexadecimal code 0x9F, in order to provide a unique character mapping in both directions.
[6] While Eight Ones is treated as a control code by IBM EBCDIC infrastructure, EBCDIC code pages from Fujitsu Siemens used on the BS2000 system frequently use it for a graphical character, most often the tilde.