DKV

Ask DKV for a definitive answer. Ask mutuelle for a definitive answer. We cannot see your insurance policy or know the INAMI codes of what you are considering claiming for.

I am not familiar with DKV and I can only assume that IS plus offers additional benefits.
However, I know that all mutuelles operate in the same basic way. You submit a claim to your mutuelle. If it is valid they pay up and if it isn't they tell you. You can then try DKV.
If you have a prescription for medicines and you have stickers from your mutuelle, you stick a sticker on the prescription and take it to a pharmacy. The pharmacist only charges you for the proportion not covered by your mutuelle.
If the medicine was given to you in the hospital then the cost will be included in the overall bill which will be dealt with directly by your mutuelle.

In general additional health care insurance only covers the part of the medical fees and medication not covered by your mutuality.
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Almost all pharmacies and also many hospitals and clinics nowadays charge the part covered by mutuality directly from the insurance company and only the remaining part from the patient. To do that they need your Belgian e-card, or if patient does not have e-card the ISI+ card.
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If the whole fee is charged from the patient you receive a special (usually green) receipt you need to deliver to your mutuality, which then reimburses you the part covered.
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Depending on your insurance policy DKV covers nothing, a part, or fully the part of the fees not covered by mutuality. The coverage is often, though not always, limited to items which are partially covered by mutuality. Most of medication issued by doctor under recipe (so over the counter medication excluded) is covered by mutuality up to 60-80%, again depending on your insurance policy DKV may cover the remaining part.
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To claim the reimbursement you need to send to DKV a copy of the receipts delivered to your mutuality and the original invoices you have received and paid.

We had something similar only seemed to come into use if hospitalised so I think it depends on your policy call them and ask.
I find on the whole the small sum we end up paying well worth it for fast service and no muttering about budgets or litigation

First stop is always mutuelle though