Source Products

Specify the source products which shall be combined to one mosaic.
Lets you choose one or multiple products from the file system.
Removes the currently selected product from the list of source products.

Target Product

Update target product: If selected, all processing parameters are taken from the output product which must already exists. Modification of all parameters is disabled in this mode. If 'Run' is pressed, the output product is updated with the given input files. If not selected, a new mosaic output product will be created.

Name: Used to specify the name of the target product.

Save as: Used to specify whether the target product should be saved to the file system. The combo box presents a list of file formats, currently BEAM-DIMAP, GeoTIFF, and HDF5. The text field allows to specify a target directory.
The target product must be saved, so you can not uncheck this option.

Open in ESA SNAP: Used to specify whether the target product should be opened in the Sentinel Toolbox. When the target product is not saved, it is opened in the Sentinel Toolbox automatically.

Coordinate Reference System (CRS)

Custom CRS: The transformation used by the projection can be selected. Also the geodetic datum and transformation parameters can be set, if possible for the selected transformation.

Predefined CRS: By clicking on the Select... button a new Select Coordinate Reference System Dialog is shown where a predefined CRS can be selected.

Orthorectification

Orthorectify input products: If selected the source products are not only reprojected to the defined CRS but also orthorectified.

Elevation model: You can select the digital elevation model which should be used for orthorectification. Either you use the internal elevation model of the product or an external DEM. If you select an external DEM and it is currently not installed on your system, a dialog will appear to enable you to download and install the DEM.
Currently there is only the GETASSE30 DEM available.

Mosaic Bounds

West: Specifies the western bound in degree which is guaranteed to be within the geographical boundary of the output product.

East: Specifies the eastern bound in degree which is guaranteed to be within the geographical boundary of the output product.

North: Specifies the northern bound in degree which is guaranteed to be within the geographical boundary of the output product.

South: Specifies the southern bound in degree which is guaranteed to be within the geographical boundary of the output product.

Pixel size X: Specifies the pixel size in X direction of the output product in map units.

Pixel size Y: Specifies the pixel size in Y direction of the output product in map units.

World Map: The world map in which the geographical boundary of the resulting output product is displayed.

Display source products: If selected the geographic boundaries of the source products are also displayed in the world map.

Variables

Defines the list of output variables.
A variable is composed of a name and a data source expression. The expression can be composed of mathematical operators, functions and symbols referencing the flags and bands defined in the input products, e.g. "0.5 * (radiance_11 + radiance_13)" or even boolean expressions such as "(radiance_13 > 50.0) AND NOT l1_flags.INVALID". By double-clicking into the expression field you can open the expression editor.
Add a band directly as variable.
Add a new variable.
Removes the currently selected variable from the list.
Moves the currently selected variable one line up.
Moves the currently selected variable one line down..

Conditions

Defines the list of valid pixel conditions.
A condition is composed of a name and a test expression. The boolean test expression can be composed of mathematical operators, functions and symbols referencing the flags and bands defined in the input products, e.g. "(radiance_13 > 50.0) AND NOT l1_flags.INVALID". By double-clicking into the expression field you can open the expression editor. By selecting the 'Outp.' toggle you can output the number of times a condition returned "true" for an output product pixel.
Add a new condition.
Removes the currently selected condition from the list.
Moves the currently selected condition one line up.
Moves the currently selected condition one line down..

Combine: If more than one valid pixel condition is given, they are combined using the condition operator which can take the values AND or OR. If the value is AND all criteria must be fulfilled to consider a given pixel in the output product.