I'm not sure exactly what you mean, but I'll try to give more details. We have our own DM class (DM_Moose) and we set our own field and domain decomposition routines:

  dm->ops->createfielddecomposition = DMCreateFieldDecomposition_Moose;                                              
  dm->ops->createdomaindecomposition = DMCreateDomainDecomposition_Moose;                                            
 

The field and domain decomposition routines are as follows (can see also at https://github.com/idaholab/moose/blob/next/framework/src/utils/PetscDMMoose.C):

static PetscErrorCode
DMCreateFieldDecomposition_Moose(
    DM dm, PetscInt * len, char *** namelist, IS ** islist, DM ** dmlist)
{
  PetscErrorCode ierr;
  DM_Moose * dmm = (DM_Moose *)(dm->data);

  PetscFunctionBegin;
  /* Only called after DMSetUp(). */
  if (!dmm->_splitlocs)
    PetscFunctionReturn(0);
  *len = dmm->_splitlocs->size();
  if (namelist)
  {
    ierr = PetscMalloc(*len * sizeof(char *), namelist);
    CHKERRQ(ierr);
  }
  if (islist)
  {
    ierr = PetscMalloc(*len * sizeof(IS), islist);
    CHKERRQ(ierr);
  }
  if (dmlist)
  {
    ierr = PetscMalloc(*len * sizeof(DM), dmlist);
    CHKERRQ(ierr);
  }
  for (const auto & dit : *(dmm->_splitlocs))
  {
    unsigned int d = dit.second;
    std::string dname = dit.first;
    DM_Moose::SplitInfo & dinfo = (*dmm->_splits)[dname];
    if (!dinfo._dm)
    {
      ierr = DMCreateMoose(((PetscObject)dm)->comm, *dmm->_nl, &dinfo._dm);
      CHKERRQ(ierr);
      ierr = PetscObjectSetOptionsPrefix((PetscObject)dinfo._dm, ((PetscObject)dm)->prefix);
      CHKERRQ(ierr);
      std::string suffix = std::string("fieldsplit_") + dname + "_";
      ierr = PetscObjectAppendOptionsPrefix((PetscObject)dinfo._dm, suffix.c_str());
      CHKERRQ(ierr);
    }
    ierr = DMSetFromOptions(dinfo._dm);
    CHKERRQ(ierr);
    ierr = DMSetUp(dinfo._dm);
    CHKERRQ(ierr);
    if (namelist)
    {
      ierr = PetscStrallocpy(dname.c_str(), (*namelist) + d);
      CHKERRQ(ierr);
    }
    if (islist)
    {
      if (!dinfo._rembedding)
      {
        IS dembedding, lembedding;
        ierr = DMMooseGetEmbedding_Private(dinfo._dm, &dembedding);
        CHKERRQ(ierr);
        if (dmm->_embedding)
        {
          // Create a relative embedding into the parent's index space.
          ierr = ISEmbed(dembedding, dmm->_embedding, PETSC_TRUE, &lembedding);
          CHKERRQ(ierr);
          const PetscInt * lindices;
          PetscInt len, dlen, llen, *rindices, off, i;
          ierr = ISGetLocalSize(dembedding, &dlen);
          CHKERRQ(ierr);
          ierr = ISGetLocalSize(lembedding, &llen);
          CHKERRQ(ierr);
          if (llen != dlen)
            SETERRQ1(((PetscObject)dm)->comm, PETSC_ERR_PLIB, "Failed to embed split %D", d);
          ierr = ISDestroy(&dembedding);
          CHKERRQ(ierr);
          // Convert local embedding to global (but still relative) embedding
          ierr = PetscMalloc(llen * sizeof(PetscInt), &rindices);
          CHKERRQ(ierr);
          ierr = ISGetIndices(lembedding, &lindices);
          CHKERRQ(ierr);
          ierr = PetscMemcpy(rindices, lindices, llen * sizeof(PetscInt));
          CHKERRQ(ierr);
          ierr = ISDestroy(&lembedding);
          CHKERRQ(ierr);
          // We could get the index offset from a corresponding global vector, but subDMs don't yet
          // have global vectors
          ierr = ISGetLocalSize(dmm->_embedding, &len);
          CHKERRQ(ierr);

          ierr = MPI_Scan(&len,
                          &off,
                          1,
#ifdef PETSC_USE_64BIT_INDICES
                          MPI_LONG_LONG_INT,
#else
                          MPI_INT,
#endif
                          MPI_SUM,
                          ((PetscObject)dm)->comm);
          CHKERRQ(ierr);

          off -= len;
          for (i = 0; i < llen; ++i)
            rindices[i] += off;
          ierr = ISCreateGeneral(
              ((PetscObject)dm)->comm, llen, rindices, PETSC_OWN_POINTER, &(dinfo._rembedding));
          CHKERRQ(ierr);
        }
        else
        {
          dinfo._rembedding = dembedding;
        }
      }
      ierr = PetscObjectReference((PetscObject)(dinfo._rembedding));
      CHKERRQ(ierr);
      (*islist)[d] = dinfo._rembedding;
    }
    if (dmlist)
    {
      ierr = PetscObjectReference((PetscObject)dinfo._dm);
      CHKERRQ(ierr);
      (*dmlist)[d] = dinfo._dm;
    }
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode
DMCreateDomainDecomposition_Moose(
    DM dm, PetscInt * len, char *** namelist, IS ** innerislist, IS ** outerislist, DM ** dmlist)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  /* Use DMCreateFieldDecomposition_Moose() to obtain everything but outerislist, which is currently
   * PETSC_NULL. */
  if (outerislist)
    *outerislist = PETSC_NULL; /* FIX: allow mesh-based overlap. */
  ierr = DMCreateFieldDecomposition_Moose(dm, len, namelist, innerislist, dmlist);
  CHKERRQ(ierr);
  PetscFunctionReturn(0);
}