In OPC Binary data type system (see OPC UA Data Type Systems), there can be length fields in a structure that determine length of an array that resides in some other field of the same structure.

The OPC UA Complex Data has an auto-complete functionality which allows you, in most cases, to omit the length fields from the StructuredData you pass to the component. With this feature, the component will use the actual length of the associated SequenceData field for the value of the length field, and the length value can be omitted, or (if present) will be ignored. This allows for easier coding and also provides robustness, because the encoded data must match the value of the length field, and the encoding would be incorrect if the value of the length fields differed from the actual length of the sequence.

The value of length field will not be ignored if you set it to -1. This is a special value that indicates that the associated sequence should not be present in the encoded data at all. Specifying -1 for the length always causes the associated sequence be omitted (its actual length is then irrelevant).