FLUENT being an unstructured solver uses internal data structures to assign an order to the cells, faces, and grid points in a mesh and to maintain contact between adjacent cells. Therefore, It does not require i,j,k indexing to locate neighboring cells. This gives you the flexibility to use the best grid topology for your problem, as the solver does not force an overall structure or topology on the grid.
In 2D, quadrilateral and triangular cells are accepted, and in 3D, hexahedral, tetrahedral, pyramid, wedge, and polyhedral cells can be used. Figure 6.1.1 depicts each of these cell types. Both single-block and multi-block structured meshes, as well as hybrid meshes containing quadrilateral and triangular cells or hexahedral, tetrahedral, pyramid, and wedge cells are acceptable. FLUENT also accepts grids with hanging nodes (i.e., nodes on edges and faces that are not vertices of all the cells sharing those edges or faces). See Section 26.2.1 for details. Grids with non-conformal boundaries (i.e., grids with multiple subdomains in which the grid node locations at the internal subdomain boundaries are not identical) are also acceptable. See Section 6.4 for details.
Some examples of grids that are valid for FLUENT are presented in Section 6.1.1. Different cell shapes and their face-node connectivity is explained in Section 6.1.2. Section 6.1.3 explains how to choose the grid type that is best suited for your problem.