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The use of alphanumeric sub IDs is highly recommended. Special characters (such as ß, ç or æ) should be submitted URL-encoded. Full stops, commas, quotation marks etc. should be avoided. If you export your sub ID data, particularly with Excel, make sure you use UTF-8 characters instead of ISO-formats.
Also, please note that the value for each smc parameter (sub ID) should not exceed 250 characters. Longer values will be shortened.