In a manner that can be customized or modified; in a way that allows for personalization.
From 'customable' plus the adverbial suffix '-ly,' which transforms adjectives into adverbs describing how an action occurs. This is an extremely rare formation, almost never seen in modern English.
This word is practically non-existent in actual usage—if you tried to say 'the interface is customably designed,' people would be confused and suggest you say 'customizable' instead, showing how not all grammatically correct words actually thrive in a language.
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