difference between server and DSN in RDOBC package

Question

I'm trying to connect to a MS SQL database via RODBC package. The following is not working :

ch = odbcDriverConnect(paste0("Driver=MSODBC;DSN=D.ad009.win.org;database=LCA;UID=LCA-User;PWD=xxxx"))

Warning messages:
1: In odbcDriverConnect(paste0("Driver=MSODBC;DSN=D.ad009.win.org;database=LCA;UID=LCA-User;PWD=xxxx")) :
  [RODBC] ERROR: state 08001, code 0, message [unixODBC][Microsoft][ODBC Driver 13 for SQL Server]Neither DSN nor SERVER keyword supplied
2: In odbcDriverConnect(paste0("Driver=MSODBC;DSN=D.ad009.win.org;database=LCA;UID=LCA-User;PWD=xxxx")) :
  ODBC connection failed

But changing the DSN to server will solve the problem :

ch = odbcDriverConnect(paste0("Driver=MSODBC;server=D.ad009.win.org;database=LCA;UID=LCA-User;PWD=xxxx"))

ch
RODBC Connection 8
Details:
  case=nochange
  DRIVER=MSODBC
  SERVER=D.ad009.win.org
  UID=LCA-User
  PWD=******
  WSID=d04x
  DATABASE=LCA

I was wondering what is the difference between DSN and server ???