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Users are encouraged to explore the C3S catalogue using the former Web API catalogue tool here: https://apps.ecmwf.int/data-catalogues/c3s-seasonal/?class=c3
The guidance given in section 2a here explains the mapping of syntax between that Web API catalogue and the CDS forms.
All the data shown at the same time in a window like the picture below, constitutes a "leaf" in the MARS "tree". This is usually guaranteed to be in a single archived file, and hence in one single tape.
Users are encouraged to combine in one single request all the data they are interested in which is contained in the same MARS "leaf"Expand title See here how a MARS leaf looks like in the ECMWF catalogue 
- Users are encouraged to devise their requests as follows:
- Do NOT ask for different start months in the same request when retrieving data from the "daily" datasets.
It is much more efficient to request one single month (e.g. November) for all the hindcast years (1993 to 2016) than requesting all 12 months from a single year - Combine in one single request all variables of interest, all levels (for pressure level fields), all product types and all leadtimes (hours or months, depending on the CDS product)
- However, note that there are some limitations in the size of a request. If you are using the CDS forms directly, you will be informed when you request is too big before submitting it. If you are using CDS API you can use the forms to discover whether the request you are trying to submit is affordable or not.
- Do NOT ask for different start months in the same request when retrieving data from the "daily" datasets.
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