Stratum levels are all about oscillator stability. They don't define technology, but roughly speaking we have the following equivalences:

• Stratum 1: the ultimate stability. Typically this requires a good Caesium beam implementation. Stratum 1 is the Primary Reference, so it is not required to measure holdover for this type of clock.
• Stratum 2: the next level down, equivalent to an SSU in the ITU specifications. Typically this requires a very good OCXO or Rb oscillator.
• Stratum 3E: is an intermediate level, typically implemented in simpler OCXO or high-end TCXO.
• Stratum 3: the next level down in the distribution network, as found in an SEC or EEC. Typically this requires a TCXO.
• Stratum 4: at the end node, typically only requires a reasonably stable crystal. Stratum 4 is not supposed to be used for holdover, therefore FFO and FFD are not relevant for this type of clock