Each Item of a Data Element of Value Representation SQ shall be encoded as a DICOM Standard Data Element with a specific Data Element Tag of Value (FFFE,E000). The Item Tag is followed by a 4 byte Item Length field encoded in one of the following two ways:
a) Explicit Length: The number of bytes (even) contained in the Sequence Item Value (following but not including the Item Length Field) is encoded as a 32-bit unsigned integer value (see Section 7.1). This length shall include the total length of all Data Elements conveyed by this Item. This Item Length shall be equal to 00000000H if the Item contains no Data Set.
b) Undefined Length: The Item Length Field shall contain the value FFFFFFFFH to indicate an undefined Item length. It shall be used in conjunction with an Item Delimitation Data Element. This Item Delimitation Data Element has a Data Element Tag of (FFFE,E00D) and shall follow the Data Elements encapsulated in the Item. No Value shall be present in the Item Delimitation Data Element and its Length shall be 00000000H.
The encoder of a Data Set may choose either one of the two ways of encoding. Both ways of encoding shall be supported by decoders of Data Sets. Data Element Tags (FFFF,eeee) are reserved by this standard and shall not be used.
Each Item Value shall contain a DICOM Data Set composed of Data Elements. Within the context of each Item, these Data Elements shall be ordered by increasing Data Element Tag value and appear only once (as Data Set is defined in Section 7.1). There is no relationship between the ordering of the Data Elements contained within an Item and the ordering of the Data Element Tag of SQ Value Representation that contains that Item. One or more Data Elements in an Item may be of Value Representation SQ, thus allowing for recursion.
Data Elements with a group of 0000, 0002, 0004 and 0006 shall not be present within Sequence Items.
Note: The use of Transfer Syntax UID (0002,0010) in particular is forbidden, since were it to differ from the Transfer Syntax of the enclosing dataset then a change in encoding would be implied, which is not allowed.
Section 7.8 specifies rules for incorporating Private Data Elements into Sequence Items.