Influx - Help to choose in between Tag and Field

I have the following columns: '**datetime**', - which gives the time when the sensor values are recorded. '**machineID**', - which gives the machine ID of the machine from where the sensor values are recorded. '**volt**', '**rotate**', '**pressure**', '**vibration**' - 4 different sensor values measured for a particular Machine at a time. Please let me know if I need to name datetime and MachineID coulumns under tag or fields. Thank you in advance.