Difference between $\mathbb{R}^n$ and $\mathbb{R}^{n_i}$

Question

Let $x_i, y_i \in \mathbb{R}^{n_i}$, where $x_i$ and $y_i$ are vectors. I know $\mathbb{R}^n$ is the space of all n-dimensional vectors, so what does $\mathbb{R}^{n_i}$ denote?