In short, you'll want to look for specific keyword phrases that have decent keyword volume and low to medium competition. (clio.com)
In a nutshell, you're restricting the use of a highly specific keyword phrase from general ads, even if this phrase is relevant. (entrepreneur.com)
Long - tail phrases are messy and unpredictable, making them hard to track, but you can still target and track a number of specific keyword phrases for your client. (audiencebloom.com)