You should enable the MDEX Engine for both Spelling Correction and Did You Mean.

If you want spelling corrected automatic phrases, the Spelling Correction feature ensures search terms are corrected before the terms are automatically phrased. The DYM feature provides users the choice to opt-in or opt-out of Automatic Phrasing.

The Oracle Commerce MDEX Engine applies spelling correction to a query before automatically phrasing the terms. This processing order means, for example, if a user misspells the query as Napa Valle, the MDEX Engine first spell corrects it to Napa Valley and then automatically phrases to "Napa Valley". Without Spelling Correction enabled, Automatic Phrasing would typically not find a matching phrase in the phrase dictionary.

If you implement Automatic Phrasing to rewrite the query using an automatic phrase, then enabling DYM allows users a way to opt-out of Automatic Phrasing if they want to. On the other hand, if you implement Automatic Phrasing to process the original query and suggest automatically-phrased alternatives, then enabling DYM allows users to take advantage of automatically-phrased alternatives as follow-up queries.