It is an extension of ISO/IEC 8859-1, similar to the Windows CP1252 in that many of the added characters are typographical punctuation marks.
[1] The following table shows the RISC OS character set.
Each character is shown with a potential Unicode equivalent in the tooltip.
EFF, a third-party supplier of RISC OS outline fonts, has a different, but similar character set.
This RISC OS Latin-1 character set was used by Electronic Font Foundry.