(Readin' time: 1m 57s)
I had a really enjoyable conversation with a prospective client yesterday, and it got me thinking about a useful angle on specializing.
The decision about how to specialize is, for many of us, a process of converting a set of unknowns into knowns.
I've mapped this process out verbally at a high level in my book Specializing Without Failure (specializingwithoutfailure.com), but I've never really mapped it out as a really concise 1-pager. 
Not until 4:40am Mountain Time today, that is!!
Here's a link where you can explore this map in more detail/resolution: my.mindnode.com/5rkeW73zxYxGyWcXpAh356r6q6xRTWfJHjN5dds2#48.8,14.2,2
I should really record a video walkthrough of the map above, but if this email happens to be landing at the right time for you, here are some notes that elaborate on the map and hopefully help you make use of it in your decision-making:
Your decision about specializing becomes a much more well-informed decision if you understand most or all of the "About the market" factors I identify. Unless you're one of those rare market-shaping thought leaders, pioneers, or influencers, the market and its needs/realities is your ultimate reality check, so understanding the market is your best way of understanding the constraints and boundaries on what you can and cannot do with a specialization.
Most of the folks I work with need to have strong alignment between the market and who they are as business owners. What's interesting or fascinating or important to you as a person? Where and how can you really put your back into serving a market? These factors help to narrow the range of possibilities unless you are a somewhat "mercenary" entrepreneur, in which case the market opportunity is a more important factor in leading you to a specialization.
Once you've decided how you want to specialize, you move into implementation, and one way to think about that is closing gaps: gaps in credibility, access, marketing footprint, and authority. How large a gap you need to close in these areas largely defines what your marketing activities for the first few years of specialized focus will look like.
Finally, in reality, some of this stuff can be worked out "on paper" and through research. And other parts of it have to be figured out experientially. And the decision can never be de-risked 100%.
At some point you have to leap and trust your ability to stick the landing.
1: It's possible that I have mapped this out as a concise 1-pager before and simply forgotten about it. I've written so extensively about this topic for ~3 years now that it's entirely possible I've done this before and simply forgotten about it! :) Anyway, this one will probably be better than its possible precursors.