online docs don't give a direct hint.

so my shot from the hip: 
selection expression - $select(), $case()  where you get a value based  on a condition
data expression - anything that sets, modifies, composes data.

this is  a quite academic view since  data expressions may contain selection expressions
and selection expressions may contain data expressions.
both variants and any mix of it  are quite common in COS 


It's good to know that DC is already well established on InterSystems' events.
But anyhow working during the last  2 years very close with a customer the message  of DC got lost.
My interpretation: we didn't reach the developers but just their managers.
So the message message to managers:

You as a manager take advantage giving your people time to use DC as a resource.

I didn't see it from this prospective before.

And YES! It's highly important to bring leaders and also local people together!
Personal contact could / will be a base of trust and confidence in each other. 

I totally agree that technology can't fix lack of education or lack of politeness or lack of interpersonal skills.
I also have no problems with beginners or not searching deep enough into documentation.
Also publishing subjects that someone finds important or new / updated
That's the purpose of a community as I understand it.
But then I'd expect something like : SOLVED, FAILED, OBSOLETE.

I did a short test fiddling arouned ^Samples.PersonI

SAMPLES>Do $SYSTEM.OBJ.ValidateIndices("Sample.Person","",1,2)                   
Checking index integrity for class 'Sample.Person'
Begin time:  08/17/2017 21:41:25
Verifying data from data map 'IDKEY' is indexed correctly...
Index 'NameIDX', entry ^Sample.PersonI("NameIDX"," CHANG,QUIGLEY H.",39), has differences between master map data and index map data.  Corrected
Index 'NameIDX', entry ^Sample.PersonI("NameIDX"," CHANG,DAN L.",40) missing.  Corrected
Index 'NameIDX', entry ^Sample.PersonI("NameIDX"," CHADWICK,OLGA E.",43) missing.  Corrected
Index 'NameIDX', entry ^Sample.PersonI("NameIDX"," CANNON,BARBARA W.",81) missing.  Corrected
Data Map evaluation complete, 200 rows checked, 4 errors found, elapsed time: .004733 seconds
Verifying data from index map "$Person" is correct...
Index map "$Person" evaluation complete, 0 errors, elapsed time: .000691 seconds
Verifying data from index map NameIDX is correct...
Index 'NameIDX', entry ^Sample.PersonI("NameIDX"," CEMPER,DAN L.",40), data differs for field 'Name' between data and index map.  Corrected
Index map NameIDX evaluation complete, 1 errors, elapsed time: .001331 seconds
Verifying data from index map SSNKey is correct...
Index map SSNKey evaluation complete, 0 errors, elapsed time: .001195 seconds
Verifying data from index map ZipCode is correct...
Index map ZipCode evaluation complete, 0 errors, elapsed time: .001255 seconds
%ValidateIndices is complete, total elapsed time: .012282 seconds

This looks good so far.

Similar using  Class %System.License. ...  you may reduce the number of allowed connections by USER
This might  be interesting in SaaS situation.

classmethod SetUserLimit(MaxLU As %Integer) as %Integer

$SYSTEM.License.SetUserLimit establishes the maximum number of license units that this Cache instance will consume. This permits a system manager to prevent a Cache instance from consuming all the license units in a MultiServer key shared among multiple Cache instances. The Method returns the former maximum value. In the absence of a call to $SYSTEM.License.SetUserLimit(max) a Cache instance may consume all available license units. 

Requires access to the %Admin Manage resource.

Using  Class %System.License. ...  you may reduce the number of allowed connections by instance

• classmethod SetConnectionLimit(MaxCon As %Integer) as %Integer

$SYSTEM.License.SetConnectionLimit establishes the maximum number of connections permitted per user on this instance. This enables a system administrator to prevent a user from making so many connections to a Cache instance that they consume all available license units. Calling $SYSTEM.License.SetConnectionLimit($SYSTEM.License.MaxConnections()) permits a user to make the maximum connections permitted but prevent transitioning to the license unit per connection state, insuring the user will consume no more than one license unit. A zero argument removes the limit. The failure to obtain a license will be indicated in the normal way. For example, a call to $system.License.Login() will return zero (0) while a failure to log in to a new terminal connection will receive a error. However, these license failures will not be reported in the console log. The method returns the former maximum value. 

The limit applies only to the local instance. If a Cache MultiServer license is shared among several instances it is still possible to exceed $SYSTEM.License.MaxConnections() over all instances. This can be avoided by setting the limit low enough on each instance that the combined connections to all instances will be unlikely to exceed $SYSTEM.License.MaxConnections(). 

$SYSTEM.License.SetConnectionLimit reqires access to the %Admin Manage resource. It is suggested that customers call this API from SYSTEM^%ZSTART.