diff --git a/Docs/source/usage/parameters.rst b/Docs/source/usage/parameters.rst
index 5824cdf8029..4d5949b8ed4 100644
--- a/Docs/source/usage/parameters.rst
+++ b/Docs/source/usage/parameters.rst
@@ -908,9 +908,12 @@ Particle initialization
       ``<species_name>.momentum_function_uy(x,y,z)`` and ``<species_name>.momentum_function_uz(x,y,z)``,
       which gives the distribution of each component of the momentum as a function of space.
 
-    * ``gaussian_parse_momentum_function``: Gaussian momentum distribution where the mean normalized momentum :math:`u = (u_{x},u_{y},u_{z})=(\gamma v_{x}/c,\gamma v_{y}/c,\gamma v_{z}/c)` and the standard deviation are given by functions of position in the input file.
-      It requires the following arguments which specify the distribution of each component of the mean normalized momentum
-      and standard deviation as a function of space.
+    * ``gaussian_parse_momentum_function``: Gaussian momentum distribution where the mean and the standard deviation are given by functions of position in the input file.
+      Both are assumed to be non-relativistic.
+      The mean is the normalized momentum, :math:`u_m = \gamma v_m/c`.
+      The standard deviation is normalized, :math:`u_th = v_th/c`.
+      For example, this might be `u_th = sqrt(T*q_e/mass)/clight` given the temperature (in eV) and mass.
+      It requires the following arguments:
 
       * ``<species_name>.momentum_function_ux_m(x,y,z)``: mean :math:`u_{x}`
       * ``<species_name>.momentum_function_uy_m(x,y,z)``: mean :math:`u_{y}`