When would this be useful? If your site has a blog with public commenting turned on, links within those comments could pass your reputation to pages that you may not be comfortable vouching for. Blog comment areas on pages are highly susceptible to comment spam. Nofollowing these user-added links ensures that you're not giving your page's hard-earned reputation to a spammy site.
SEM is the wider discipline that incorporates SEO. SEM includes both paid search results (using tools like Google Adwords or Bing Ads, formerly known as Microsoft adCenter) and organic search results (SEO). SEM uses paid advertising with AdWords or Bing Ads, pay per click (particularly beneficial for local providers as it enables potential consumers to contact a company directly with one click), article submissions, advertising and making sure SEO has been done. A keyword analysis is performed for both SEO and SEM, but not necessarily at the same time. SEM and SEO both need to be monitored and updated frequently to reflect evolving best practices.
Digital marketing channels and traditional marketing channels are similar in function that the value of the product or service is passed from the original producer to the end user by a kind of supply chain. For instance, a typical digital marketing channel is email. Organization can update the activity or promotion information to the user by subscribing the newsletter mail that happened in consuming. In addition to this typical approach, the built-in control, efficiency and low cost of digital marketing channels is an essential features in the application of sharing economy.
Tablet - We consider tablets as devices in their own class, so when we speak of mobile devices, we generally do not include tablets in the definition. Tablets tend to have larger screens, which means that, unless you offer tablet-optimized content, you can assume that users expect to see your site as it would look on a desktop browser rather than on a smartphone browser.
The digital marketer usually focuses on a different key performance indicator (KPI) for each channel so they can properly measure the company's performance across each one. A digital marketer who's in charge of SEO, for example, measures their website's "organic traffic" -- of that traffic coming from website visitors who found a page of the business's website via a Google search.