UNIQUECOUNTER is a special variable available in the Windows File System based modules and connectors which generates a counter when the file is already existing.

 

%UNIQUECOUNTER% = initially not existing when file doesn't exist
%UNIQUECOUNTER3% = initially not existing when file doesn't exist, it is formatted by x (3) digits (e.g. 000)
%UNIQUECOUNTER3+7% = initially not existing when file doesn't exist, it is formatted by x (3) digits and starting from y (7) (e.g. 007)
%UNIQUECOUNTER3+7I% = (Inclusive) same as before but initially inclusive (so first file it will come directly with 007 rather than empty) -> this is your case
%UNIQUECOUNTER3+7+_% = existing or not initially according the usage of I which can be added or not but using the _ character as separation from the previous filename (e.g. _007). This is for cases when you don't want the extra char when the counter isn't used (example without I so counter not used initially) because if you put the _ in the filename then the initially filename will just end in _ being without the counter.