Auto populate service text based on service no in CJ20n

hello all,

In CJ20N under service specifications,

if i enter service no and hit enter button (as shown in my screen shot)

the corresponding service text, units are automatically getting populated (and asking me to enter quantity)

please let me know how this is getting populated,

my intention is based on this i want to populate gross price (maintained in some ztable) there by not allowing the end user

to enter random price..

Note : moderators please do not block this thread, my intention is to get some hint from the experts not a step by step procedure..

Thanks in advance..