Basic concept of sending and receiving data with BLE

[Soojin]

HI!
I've done the basic Blinky and UART examples, and now have to move on to programming something on my own.
( I have to send EEG signals from one BLE board to another/notebook )
And before that I'm trying to fully understand the basic structure and concept of BLE communication. 
I've heard that when sending and receiving data with BLE, nRF52840 DK have a structure and a parameter of its own.

Can someone check if I'm understanding things correctly? 

1. Sending data > Writing data in the GATT server. (Slave, Peripheral, GATT server)

2. Receiving/Reading data > Reading data from the GATT server (Master, Central, GATT client )

3. So if I want to send data from one BLE to another(without using UART or NUS), I need codes to make a service of my need. (I'm not sure if this is the right way to say it)

Additionally, I have one concept that I find confusing 

4. What exactly is a 'handler'? Can I understand it as some kind of function??

Always thank you all for your help:)

[Amanda Hsieh]

Hi, 

Sorry for the delay. The support staff is reduced during the summer holidays, and you may experience delayed answers.

1. Sending data > Writing data in the GATT server. (Slave, Peripheral, GATT server)

2. Receiving/Reading data > Reading data from the GATT server (Master, Central, GATT client )

Please see the data transfer: 

3. So if I want to send data from one BLE to another(without using UART or NUS), I need codes to make a service of my need. (I'm not sure if this is the right way to say it)

Not sure what kind of data you want to send over BLE. Hope this ble-services-a-beginners-tutorial might help. 

4. What exactly is a 'handler'? Can I understand it as some kind of function??

It is a function for handling events from the module. 

-Amanda H.