A dimension takes several codes joined by +, so {"REF_AREA": "JP+XM"} asks
for both in one request rather than two. A dimension left out of the key
matches every code, which is how to ask which countries a flow covers:
fetch it with only FREQ pinned and read the series keys.

Returns the invariant part of the key once under "key", and observations as
[period, value] pairs grouped into "series" by whichever dimensions vary.
"names" gives the label for every code in the response, so a series keyed
XDC or CP01 can be read without asking search_codes again.
"units" says what the numbers are measured in, including any multiplier:
read it before comparing or converting. "range" is the period span actually
returned; periods with no value are omitted and counted under "empty".
"total" counts what the provider sent, which is the whole series unless
"truncated" says it is a floor: only the newest observations are fetched,
so a longer history exists and was not counted. Narrow start/end to read
it.

An empty response echoes the key it was given and says so under "note".
Any single entry in the key can empty a result and they all look alike, so
drop a dimension to find which one.

"revisions" lists each period whose value changed and between which two
vintages. Vintages that do not carry the key at all are listed under
"no_data": coverage varies between them, not only values.

These are a 14:15 CET fixing on TARGET business days, not a traded spot
rate, and a closed day returns the last publication before it. Every ECB
rate is quoted against the euro, so a pair without EUR is a cross derived
from two fixings — the response marks it "derived": true. Report that.