What is the "Z" parameter in "GPy.mappings.kernel module"?

[AlbanMor]

Hi,

I'm trying to use an RBF kernel as a mean function in my GPR by using the GPy.mappings.kernel module but I can't seem to understand what is the "Z" parameter that is expected in this module.

"""
Mapping based on a kernel/covariance function.

.. math::

   f(\mathbf{x}) = \sum_i \alpha_i k(\mathbf{z}_i, \mathbf{x})

or for multple outputs

.. math::

   f_i(\mathbf{x}) = \sum_j \alpha_{i,j} k(\mathbf{z}_i, \mathbf{x})


:param input_dim: dimension of input.
:type input_dim: int
:param output_dim: dimension of output.
:type output_dim: int
:param Z: input observations containing :math:`\mathbf{Z}`
:type Z: ndarray
:param kernel: a GPy kernel, defaults to GPy.kern.RBF
:type kernel: GPy.kern.kern

"""

Any help would be appreciated!