Q:

In magento 2 - What mean of 'is_used_for_customer_segment' in customer_eav_attribute table?

In magento 2 EE, What mean of 'is_used_for_customer_segment' in customer_eav_attribute table (explain when set it's value is true and false) ?

A:

is_used_for_customer_segment is used only in Magento EE.

When is_used_for_customer_segment is true they display inside customer
  segements select option backend otherwise its not display in customer segementation section.

Customer segmentation is useful feature for magento enterprise for marketing perspective.
Customer segmentation is the simply termonolgoy of divide a customers into groups of individuals that are similar in specific ways relevant to marketing, such as based on email id simillarilty(@gmail.com), geoip , gender, hobby, and so on. 
Using segmentation allows magento admin to target groups in effectively manner and allocate marketing resources to best possible manner.
Using create target groups of customers using a variety of customer characteristics as well as purchase and browsing behavior.
Segmentation is used to personalize content to those customer segments across the website.
You can find this attribute inside 

Stores -> Attributes -> Customer

Now go to customer attribute row,

Now go to customer segmentation attribute value inside
Click on left side menubar,

Customers -> Segments

In above image click on green arrow,
you have to display select box with please choose conditions to add and all of your customer segmentation is enable in customer attribute are display here.

