Element API + checkboxes
You should be able to foreach the $block->product to include the selected values (or labels)

return [
    'endpoints' =&gt; [
        'fullmembers.json' =&gt; [
            'elementType' =&gt; 'User',
            'criteria' =&gt; ['group' =&gt; 'fullMembership'],
            'transformer' =&gt; function(UserModel $member) {
                $businessCategories = [];
                foreach ($member-&gt;businessTypeMarketSectorsProducts as $block) {
                    switch ($block-&gt;type-&gt;handle) {
                        case 'businessCategory':
                            $productValues = [];
                            foreach ($block-&gt;product as $product) {
                                if ($product-&gt;selected) {
                                    $productValues[] = $product-&gt;value;
                                }
                            }
                            $businessCategories[] = [
                                'businessType' =&gt; $block-&gt;businessType-&gt;value,
                                'marketSector' =&gt; $block-&gt;marketSector-&gt;value,
                                'product' =&gt; $productValues
                            ];
                            break;
                    }
                }

                return [
                    'id' =&gt; $member-&gt;id,
                    'companyName' =&gt; $member-&gt;membersCompanyName,
                    'addressLine3' =&gt; $member-&gt;membersAddressLine3,
                    'lat' =&gt; $member-&gt;memberAddress-&gt;lat,
                    'lng' =&gt; $member-&gt;memberAddress-&gt;lng,
                    'email' =&gt; $member-&gt;email,
                    // 'profile' =&gt; '/directory/company/' . $member-&gt;id . '/' . $member-&gt;username,
                    'profile' =&gt; UrlHelper::getUrl("directory/company/{$member-&gt;id}"),
                    'businessCategories' =&gt; $businessCategories
                ];
            },
        ]
    ]
];