--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/Back-end-selection	Sun Mar 21 06:24:52 2021 +0000
@@ -0,0 +1,46 @@
+fc-simtool and fc-uicc-tool can operate on SIM cards via any of the following 3
+access paths:
+
+* The card to be operated on can be inserted into a smart card reader/programmer
+  supported by pcsc-lite software stack, such as HID Omnikey 3121 and 6121 card
+  readers sold by Sysmocom webshop;
+
+* The card can be inserted into a much simpler serial SIM reader - see the
+  Serial-SIM-readers article;
+
+* The card can be sitting inside a Calypso phone or development board - see the
+  SIM-in-Calypso-phone article.
+
+In the PC/SC and serial SIM reader use cases, you run fc-simtool or fc-uicc-tool
+directly, with the needed options to select the right target.  In the more
+special case of operating on SIM cards inside Calypso GSM devices, you have to
+use the fc-simint front end described in the SIM-in-Calypso-phone article.
+
+When running fc-simtool or fc-uicc-tool directly to operate on a card inserted
+into a dedicated reader/programmer, use the following options to select the
+correct target:
+
+-d /dev/ttyXXX
+
+	This option selects the serial SIM reader back end, operating on the
+	named serial port.  See the Serial-SIM-readers article for additional
+	speed enhancement options.
+
+-p N
+
+	This option selects the pcsc-lite back end, operating on reader number
+	N.  If you have only one PC/SC-supported card reader on your system,
+	the correct option is -p0.  If you have multiple readers, use
+	fc-pcsc-list utility to list them and figure out which number is which.
+
+There is no default: if you run fc-simtool or fc-uicc-tool with neither -d nor
+-p, you will get an error.
+
+Each of the two target selection options runs a different back end program
+behind the scenes: -d runs fcsim-serial-be, -p runs fc-pcsc-backend.  This
+separation allows for subset installations: if someone is only interested in
+serial and/or Calypso back ends and does not have libpcsclite installed, they
+can omit the pcsc back end, in which case fc-pcsc-backend won't be present and
+therefore -p option won't work.  One can similarly omit the serial and Calypso
+back ends, although the Mother does not currently know of any practical system
+where they would pose compile-time difficulties.