The System Variable ⎕MC contains the character used to replace a Unicode or other character which cannot be represented in APLX. By default, it is set to a question mark, but you can set it to any character in ⎕AV. It can be localized in the header of a function or method.
See ⎕UCS for an example, and for more information on translating to and from Unicode.
Note that, as well as translation using ⎕UCS, the character specified in ⎕MC is also used when translating Unicode text received anywhere within APLX. This can include Unicode text which has been:
pasted from the clipboard
returned by an external object using the .Net, Java or other interface