what are the differences between `event.sender.send` and `webContents.send` in electron?

As this code snippet showed, both method can send event to the render process. I am wondering what are the differences between line A and line B in the code?

ipcMain.on('async', (event, arg) => {          
    console.log(arg);      
    event.sender.send('async-reply', 2); // line A
});


ipcMain.on('sync', (event, arg) => {          
    console.log(arg);
    event.returnValue = 4;
    mainWindow.webContents.send('ping', 5); //line B
});