a.php

public function index() {
  $this->validate($request, [
      "equipmentType" => 'bail|required|in:pac,cto,roteamento,ponto|string',
      "equipmentId" => [
        "required", 
        Rule::exists($request->get('equipmentType')->where(function ($query) use ($request) {
            $equipmentType = $request->get('equipmentType');
            if($equipmentType === 'pac') {
              $column = 'cdpac'
            } else if ($equipmentType === 'cto') {
              $column = 'cdcto'
            }
            $query->where($column, $request->get('equipmentId'));
        }),
      ]
  ]);
}