The Field Descriptor Codes (FDCs) are used during map processing. They are specified in the map during map creation or they can be used during program execution.

When used during map creation, FDCs are entered in either the MAPSTART definition, the MAPF definition, or with the ATTRIBUTE UPDATE function of the UMAP utility. During program execution, they are specified in individual FCTE entries.

The permissible FDCs are defined in the following table. The FDCs are grouped in six categories. The FDCs in each category are mutually exclusive. Note that if more than one code from any one group is specified, the last one encountered takes precedence.

FDC	Field Type	Definition
(1)
B	All	Blink. (If the device type supports the feature.)
D	All	Display with normal intensity
H	All	High intensity display
L	Variable only	Light pen selectable
N	All	Non-displa
V	All	Video. Reverse the display in the field if the video feature is supported by the device.
X	All	Underline the field if the feature is supported by the device.
(2)
P	All	Protected. Do not allow data to be entered in this field.
S	All	Skip. Do not hardware tab stop on this field.
U,T	All	Unprotected tab. Do allow this field to be modified by setting the hardware tab.
(3)
O	All	Optional. Data entry is optional in this field.
R	Variable only	Required. Data entry is required in this field.
(4)
K	All	Turn off the modified data tag in this field.
M	Variable only	Turn on the modified data tag in this field. Note: M is supplied as a convenience to 3270 users only. It’s use by application devices other than 3270 may yield unpredictable results. When conversation mode is used, terminal I/O should eliminate the need for using the modified data tag to store data at the terminal.
(5)
Y	All	Skip can be used. The field can be used with a skip character. The skip character will not overlay a field already on the screen. Used for split screen mapping. (Constant fields require no ending attribute.)
Z	Variable only	Skip can not be used. The field can not be ended with a skip character because the attribute byte will overlay a field already on the screen. Used for slip screen mapping.
(6)
E	Variable only	Erase the field.