SEM is a broader term than SEO, and is used to encompass different options available to use a search engine’s technology, including paid ads. SEM is often used to describe acts associated with researching, submitting and positioning a website within search engines.  It includes things such as search engine optimization, paid listings and other search-engine related services and functions that will increase exposure and traffic to your Web site. 
QUOTE: “The preferred domain is the one that you would liked used to index your site’s pages (sometimes this is referred to as the canonical domain). Links may point to your site using both the www and non-www versions of the URL (for instance, and The preferred domain is the version that you want used for your site in the search results.” Google, 2018

Google Ads (formerly Google Adwords) is the search provider most commonly used for this strategy. With this tactic, brands conduct keyword research and create campaigns that target the best keywords for their industry, products, or services. When users search for those keywords, they see the custom ads at the top or bottom of SERPs. The brand is charged each time a user clicks on the ad.

Robots.txt is not an appropriate or effective way of blocking sensitive or confidential material. It only instructs well-behaved crawlers that the pages are not for them, but it does not prevent your server from delivering those pages to a browser that requests them. One reason is that search engines could still reference the URLs you block (showing just the URL, no title or snippet) if there happen to be links to those URLs somewhere on the Internet (like referrer logs). Also, non-compliant or rogue search engines that don't acknowledge the Robots Exclusion Standard could disobey the instructions of your robots.txt. Finally, a curious user could examine the directories or subdirectories in your robots.txt file and guess the URL of the content that you don't want seen.
What would be the purpose of/reason for moving back to a different url? If its been a few years, I’d leave it alone unless you watched everything decline since moving to the main url. Moving the forum to a new url now would probably be a bit chaotic, not only for your main url but for the forum itself…. Only reason I could imagine myself moving the forum in this scenario would be if all those links were really awful and unrelated to the url it currently sits on…
In particular, the Google web spam team is currently waging a PR war on sites that rely on unnatural links and other ‘manipulative’ tactics (and handing out severe penalties if it detects them). And that’s on top of many algorithms already designed to look for other manipulative tactics (like keyword stuffing or boilerplate spun text across pages).
Website-specific crawlers, or software that crawls one particular website at a time, are great for analyzing your own website's SEO strengths and weaknesses; they're arguably even more useful for scoping out the competition's. Website crawlers analyze a website's URL, link structure, images, CSS scripting, associated apps, and third-party services to evaluate SEO. Not unlike how a website monitoring tool scans for a webpage's overall "health," website crawlers can identify factors such as broken links and errors, website lag, and content or metadata with low keyword density and SEO value, while mapping a website's architecture. Website crawlers can help your business improve website user experience (UX) while identifying key areas of improvement to help pages rank better. DeepCrawl is, by far, the most granular and detailed website crawler in this roundup, although Ahrefs and Majestic also provide comprehensive domain crawling and website optimization recommendations. Another major crawler we didn't test is Screaming Frog, which we'll soon discuss in the section called "The Enterprise Tier."
Thank you for the great checklist. It is really useful and gave us a couple of new hints on optimization process. We’ve been using WebSite Auditor and found it exremely helpful in site’s structure and content analysis, bringing all the statistical information on the validation errors, social mentions, duplicate stuff, etc. under one roof. Still there is so many different SEO information you sometimes cannot understand what to start with. Fortunately you answered this question. Thank you again!