Hi Viajaz and welcome!
If i’m correct there is no major difference between the two except for their intended use case. In other words; this is primarily a cosmetic thing. Please read this explanation for further information:
https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_pool_usage
In short; with the ‘pool’-directive you can use ‘au.pool.ntp.org’ and with the ‘server’-directive is is best to use ‘[0123].au.pool.ntp.org’, in your example.
There is one notable difference though, that I would like to mention. Only 2[.X].pool.ntp.org provides IPv6-addresses (AAAA-records). So, if you want IPv6 and also prefer to use the recommended ‘pool’-directive, just put 2.au.pool.ntp.org there, not au.pool.ntp.org. Some software does this by default nowadays.
Hope this helped!