Thanks to both of you for hugely helpful answers! 

Now, the last bit ... 

Where is the value -- or the variable, more accurately -- associated with a 
characteristic defined? So that I know which one to update. :-) 

dg

> On Nov 28, 2016, at 4:30 PM, Christopher Collins <[email protected]> wrote:
> 
> As long as you specify one of the BLE_GATT_CHR_F_NOTIFY or
> BLE_GATT_CHR_F_INDICATE flags in the characteristic definition, the
> characteristic will be subscribable.
> 
> When a peer subscribes to a characteristic, the stack signals this event
> to the application via the GAP event callback associated with the peer.
> When a peer unsubscribes, this is signalled via the same mechanism.  For
> rationale on why the GAP event callback is used here, see this email:
> https://lists.apache.org/thread.html/8706f7914d2b716a02c25bdfc57fe942f7c7fca82446dd3523014d43@%3Cdev.mynewt.apache.org%3E
>  
> <https://lists.apache.org/thread.html/8706f7914d2b716a02c25bdfc57fe942f7c7fca82446dd3523014d43@%3Cdev.mynewt.apache.org%3E>
> 
> As Mike indicated, your application tells the stack that a
> characteristic's value has changed via the ble_gatts_chr_updated()
> function.  It is up to the application to actually change the value
> prior to calling this function.
> 
> If you are not sure which handle is associated with a particular
> characteristic, you can perform a lookup by
> service-characteristic-UUID-pair using the ble_gatts_find_chr()
> function.

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