It isnt a flag. It's a trigger option (you can figure out the names of options by creating a trigger using the dialog, and then copy it and paste it into a text editor, then just look to see what the option is named (and what value you want)). Obviously in this case it would be the send_to value, you just have to figure out what number (I believe its 10).
Technically, it 'would' be a flag, except we kept requesting features to be added to aliases, triggers, etc. and Nick was reluctant to modify the existing addalias and addaliasex functions to including all the extra fields and other settings. In some respects I understand it, but it was still annoying.
Flags are more for toggling things (the checkboxes). You wouldn't have a flag for each sendto, since it's one option. It would be a parameter, yes, but not a flag.
I mean, yeah, you could offset the binary value of the sendto to not conflict and then encode it into a bunch of constants, but that doesn't really fit with the flag mentality (a single bit for a single value).