sr_put_userdata(fd, offset, data, for_ioctl) is the counterpart to sr_get_userdata() and (like sr_get_userdata()) does one of two things:
1) Copies data from a buffer (to be more specific, a chain of accessors) within the network service (this process) to a buffer within the user process. This can be either ioctl data (in which case, for_ioctl is TRUE) or read/write data (for_ioctl is FALSE). For example, udp_ioctl() (indirectly) calls sr_put_userdata() to give configuration data to a user process. Also, udp_packet2user() (indirectly) calls sr_get_userdata() to pass data to the user process.
2) Sends a message to the FS. For example, if a read is attempted on a udp file descriptor before the file descriptor is configured, reply_thr_put() is called, which then (indirectly) calls sr_put_userdata(), passing in EBADMODE for the parameter count.
In my opinion, like sr_get_userdata(), this should have been made into two functions. As it is, it is too confusing.