Many Informix applications use the function keys F1-F10 to implement special functions directly. Most PC-based emulators are not set up correctly to send these keys to the Informix application.
To set an emulator up properly, map the keys F1 thru F10 as follows:
F1 - \eOy F2 - \eOP F3 - \eOQ F4 - \eOR F5 - \eOS F6 - \eOt F7 - \eOu F8 - \eOv F9 - \eOl F10 - \eOw
where "\e" is whatever string the emulator needs to see in order to send an escape. These settings correspond to the termcap entries "k0", "k1", ... "k9", as noted in the Informix document "IBM Informix 4GL Reference Manual" under "Modifying termcap and terminfo", the "Extending Function Key Definitions" section.
Any logins that wish to use Informix applications should set "TERM=vt100". Note that the Informix termcap in $INFORMIXDIR/etc/termcap has some whack-oh entries for VT100 keys so don't use it. The regular Linux VT100 termcap definition seems to work OK.