This is a space of vector valued Siegel modular forms with real representation $\mathrm{Det}^{ 10}\otimes\mathrm{Sym}^{ 8 }$. The following table gives the dimensions of various subspaces of $M_{10, 8}(\mathrm{Sp}(4, \mathbb{Z}))$.
Total | 3 |
Non cusp | 1 |
Cusp | 2 |