This command is MKII-only. It loads support for one or more font encodings not already preloaded in the core. This is seldom needed (if ever).
ConTeXt core already uses:
If you had a need to use Cyrillic encoding t2c for example, you could use
\useencoding[t2c] \usetypescript[antykwa-torunska][t2c] \setupbodyfont[antykwa-torunska]
However t2c is already preloaded with enco-cyr.
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