How can I request certain data from the server using UDP?

I am interested in how to correctly request data via the client. For example, the server has two types of data. I would like to ask for data under the letter a

    Client                           Server
       |                               |
       |                               |         for example an array
       |                               |     a - data (1,2,3,4,5)
       |          how to ask?          |    /
    sendto - - - - - - a - - - - -> recvfrom  
       |                               |    \
       |                               |     b - data (0,0,0,2,1)
       |                               |
       |                               |
   recvfrom <- - - - - a - - - - - - sendto
       |
       |
       |
       a (1,2,3,4,5)

At the moment I send data and get it like this

#include <WinSock2.h>
#include <Ws2tcpip.h>


amountRead = recvfrom(sock, (char*)&array_a, sizeof array_a, 0, (LPSOCKADDR)&sAddr, &sizeAddr);
if (amountRead == SOCKET_ERROR) {
    std::cout << " WARNING: " << "\t" << std::endl;
}
else {
    std::cout << " GOOD: " << "\t" << std::endl;
}

sendto(sock, (char*)&array_a, sizeof array_a, 0, (LPSOCKADDR)&sAddr, sizeAddr);

As you understand now there is no second data, all because I do not know how to make a check on the server at the request of the client and also do not know what the client should send to the server to get certain data.