From The x3270 Wiki

Historically, ASCII refers to the American Standard Code for Information Interchange, a 7-bit character code covering basic Latin letters, numbers, symbols and control codes. The first 128 Unicode code points, U+0000 through U+007F, correspond to ASCII code points.

In the context of the x3270 family, the term ASCII is used more loosely to refer to the character encoding of the workstation's current locale. This is learned dynamically by the emulator at start-up, or can be overridden by the utf8 resource. POSIX-based workstations typically use UTF-8 encoding; Windows is usually configured to use one of the Microsoft code pages, such as Windows-1252.

So when the x3270 family documentation mentions "translated from ASCII", "translated to ASCII", or "interpreted as ASCII", it really means that it is using this local encoding.