It is a good Idea that search engines have clear cut policies on do follow and other types of links. Any link from a relevant page with relevant contnets will add more value ,but SE's also say that they must be natural, but we all do signature postings in forums and we do commenting which is like forcibly pushing a link to be read by the S.E why don't they simply say "there can not be no do follow in signatures and comments instead of giving the webmaster a chance to decide if it is to be a do follow or external follow.? is there any reason which is beyond my thinking ?
search engines do have clear cut policies. however, even the constitution has holes for which we need time to time amendments. SE algo also goes through routine updates to get better day by day
SE can differentiate the links witch in signatures and lower the priority of it. But still a good methord of link building