Configuration Setting document - MIME - Basics tab
The primary and secondary character group settings enable IBM® Lotus® Domino™ to select a character set encoding to apply during message conversion when automatic tests to determine the language used in the message are inconclusive.

If MIME tags in an inbound message do not specify character set information, Domino attempts to match byte range and control code patterns in the text to determine the character set. However, this process of automatic character set detection is not always sufficient to distinguish between character sets that contain similar byte patterns. If Domino cannot distinguish whether a given message uses one of two different character sets, for example, EUC-KR (a Korean character set) or GB2312 (a Simplified Chinese character set), it refers to the priority order assigned to the primary and secondary character set groups to determine which character set to use in converting the message to Notes rich text.

Similarly, for outbound messages, if byte patterns in the message are not sufficient to enable Domino to distinguish the correct language, Domino refers to the same priority order to assist in selecting the character set to use when converting an IBM® Lotus® Notes® rich text message to MIME for SMTP transport.
FieldEnter
Primary character set groupChoose the language or region appropriate for your organization, for example, Western.
Secondary character set groupsChoose the language or region(s) appropriate for your organization, for example, English.