In the documentation : http://www.quickfixj.org/quickfixj/usermanual/1.5.0/usage/configuration.html

ValidateUserDefinedFields If set to N, user defined fields will not be rejected if they are not defined in the data dictionary, or are present in messages they do not belong

With default value : N

But in the code line 76 the variable checkUserDefinedFields is initialized to true.

62 public class DataDictionary {
63 private static final String FIXT_PREFIX = "FIXT";
64 private static final String FIX_PREFIX = "FIX";
65 public static final String ANY_VALUE = "_ANY_";
66 public static final String HEADER_ID = "HEADER";
67 public static final String TRAILER_ID = "TRAILER";
68 private static final String MESSAGE_CATEGORY_ADMIN = "admin".intern();
69 private static final String MESSAGE_CATEGORY_APP = "app".intern();
70
71 private static final int USER_DEFINED_TAG_MIN = 5000;
72 private static final String NO = "N";
73 private boolean hasVersion = false;
74 private boolean checkFieldsOutOfOrder = true;
75 private boolean checkFieldsHaveValues = true;
76 private boolean checkUserDefinedFields = true;