XA collection contains datasets and other collections, and sometimes helper objects like places and instruments. Datasets in turn may contain other datasets.
Note: all collections gives a longer list than the one on this page.

XTimes is for datasets where date/time is an essential part of the data, such as in meteorology or oceanography. It is not for reproduceable experiments such as common in physics and chemistry, and is not used for publication dates.

XPlaces is for datasets where geolocation is essential, such as in meteorology or oceanography. It is not for reproduceable experiments such as common in physics and chemistry, and is not for e.g. the laboratory where these experiment are conducted.

XNetCDF is a data format particularly suited for multidimensional blocks of numbers, including metadata about scale, units etc. For some NetCDF datasets, we also provide CDL, NcML and CSV versions.

X For netCDF datasets we offer, besides the usual download, OPeNDAP access. Benefits are: view internal metadata hidden in the data files, make slices and subsamples of data, programmatic access to data with APIs for Java, Python, Matlab, etc. More info: THREDDS, OPeNDAP intro.

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Note to uploaders: 64-bit integers in netCDF

64-bit integers (int64 or long, depending on your lingo) are allowed in netCDF. But unfortunately, they are currently not supported in OPeNDAP. So, when you upload netCDF with an int64 variable, it will be perfectly usable for those who download the file for local processing but it will not be accessible through the OPeNDAP protocol.
It is therefore advised NOT to use int64 but instead use float64 (aka double), or int32 if feasible (e.g. for dates in seconds after 1970-01-01T00:00:00Z you are safe until 2038).