Currently, I am developing a continuous model. I want to assign the LU to each catchment; but the classifications are Open Land, Grass Land, Light Forest, Heavy Forest, and Crops up to shoulder height. Could you please tell me where exactly this LU is use

Land Use is generally used for: a. Water quality purposes and b. Runoff purposes (to calculate Curve Number (also known as CN) when the SCS equation is used).

Water Quality purposes:

For any hydrograph commands (e.g., StandHyd, NasHyd, etc.), at the bottom of the properties tab, there is an option to define the land use percentage for that subcatchment (e.g., StandHyd, NasHyd, etc.):

To define the land use percentage for that sub-catchment, we first need to add water quality data to our model.
We need to open the Resource Library (under the simulation ribbon) and add water quality data:

The next step is to define/edit/add/remove the land use and its Total suspended solids (TSS) and Total phosphorus (TP). Then, we need to click on “Add to Model” to add that water quality data to our model. After that, we can then go back to that sub-catchment and its properties tab, click on “…” next to land use, and define how much of the area of that sub-catchment is covered by any of the land uses.
When running the model, we need to make sure that we checked the water quality option to include water quality analysis in the modelling:

For runoff purposes:

CN is calculated based on the land use of our sub-catchments, soil types, and Antecedent Soil Moisture Condition (AMC). If we happen to have soil shapefile and land use shapefiles, we can use the “calculate CN” option under the GIS tools ribbon using the Map view of Visual OTTHYMO.
If we want to use conceptual modelling in a Schematic view, land use would affect different parts of our sub-catchments (i.e., land use affects TIMP and XIMP). The following table recommends TIMP and XIMP values, based on land use, for the macro-level studies.

Land Use is also important in the CN of our sub-catchments. If we want to use conceptual modelling in the Schematic view, we need to manually add CN for our catchments. In StandHyd and only if the SCS equation is used for Loss, will CN be asked to be added.

In VO-SWMM:

We first need to define the land uses and their associated:

After defining the land uses, we need to go through the properties tab for our sub-catchment, select “…” next to Land Use, and add the % area for each land use we defined.

The following links can be helpful as well:

Land Use, Calculating CN, Assign Land Use for Hydrograph Commands.