From InSim.txt
ISP_BTT, // 47 - info : sent after typing into a button
As you can see, IS_BTT is an info packet, which means it is always coming from LFS and never sent to it. LFS will send this packet when someone confirms (via "Ok") their input into a custom dialog window. This dialog is created by sending an IS_BTN with TypeIn set:
// TypeIn byte : if set, the user can click this button to type in text.
// Lowest 7 bits are the maximum number of characters to type in (0 to 95)
// Highest bit (128) can be set to initialise dialog with the button's text
// On clicking the button, a text entry dialog will be opened, allowing the specified number of
// characters to be typed in. The caption on the text entry dialog is optionally customisable using
// Text in the IS_BTN packet. If the first character of IS_BTN's Text field is zero, LFS will read
// the caption up to the second zero. The visible button text then follows that second zero.
// Text : 0-65-66-0 would display button text "AB" and no caption
// Text : 0-65-66-67-0-68-69-70-71-0-0-0 would display button text "DEFG" and caption "ABC"
Note that to actually see the dialog, users have to click the button.