Mike,
321 stainless steel
Titanium is used as stabilization. In order for it to do that effectively it needs to be present in volume percents that will allow it to stabilize those elements it is intended to stabilize. In other words, effectively render them not available for bonding with Cr, generally.
So, in order to prevent Cr nitrides and carbides it is required to be present at levels 5 times greater than those two elements combined. The 5 X is the min the .70 is the maximum required.