There have been a number of inquiries with regards to the custom field column definitions. So here they are:
MD_AGGREGATION_TYPE_ENUM: Specifies the roll up type for a custom field.
MD_ENT_TYPE_UID: Specifies the type of entity (project, task, or resource) for the custom field.
MD_LOOKUP_TABLE_UID: Specifies the GUID of the lookup table to use for the custom field.
MD_PROP_CHECKOUTBY: GUID of the user who has the custom field checked out for modification.
MD_PROP_CHECKOUTDATE: Date the custom field was checked out for modification.
MD_PROP_DEFAULT_VALUE: GUID of the default value in a lookup table for a custom field.
MD_PROP_FORMULA: Formula that calculates the value of a custom field.
MD_PROP_GRAPHICAL_INDICATOR: Definition of a graphical indicator for a custom field.
MD_PROP_GRAPHICAL_INDICATOR_TOOLTIP: Specifies whether the graphical indicator has a tool tip.
MD_PROP_ID: Integer ID of the enterprise custom field.
MD_PROP_ID_SECONDARY: Assignment custom field ID for resource and task custom fields.
MD_PROP_IS_LEAF_NODE_ONLY: Specifies whether the custom field value must be a leaf node in the lookup table.
MD_PROP_IS_REQUIRED: Specifies whether the custom field is required when the entity (project, resource, or task) is created.
MD_PROP_MAX_VALUES: Specifies whether the custom field has a single value or uses a multi-value lookup table.
MD_PROP_NAME: Name of the custom field.
MD_PROP_PROJ_SUMM_GRAPHICAL_INDICATOR: Definition of the project summary graphical indicator for the custom field.
MD_PROP_ROLLDOWN_TO_ASSN: Specifies whether the resource or task custom field rolls down to assignments.
MD_PROP_SUMM_GRAPHICAL_INDICATOR: Definition of the custom field graphical indicator for the summary task.
MD_PROP_TYPE_ENUM: Type of the custom field.
MD_PROP_UID: GUID of the custom field.
MD_PROP_UID_SECONDARY: Assignment custom field GUID for resource and task custom fields.
MD_PROP_USE_FOR_MATCHING: Specifies whether to use the resource custom field for matching generic resources.
And what is CUSTOM_FIELD_UID and how does that differ from MD_PROP_UID ?