A method for processing media data encapsulated as a container file (90, 722) comprising: receiving the container file (90, 722) according to a first format, the container file (90, 722) including or referring to the media data comprising at least two media data units, wherein a media data unit is a sample included in a track according to ISOBMFF or its derivatives; receiving a metadata item according to a second format different from the first format, the metadata item being linked with one or more of said at least two media data units, the container file (90, 722) including or referring to the metadata item, wherein the metadata item is either included in the container file or the metadata item is referenced in the container file, and wherein the metadata item is identified and characterized in a Meta Box inserted at a track level according to ISOBMFF or its derivatives; parsing the container file for scope information, wherein the scope information indicates said one or more of said at least two media data units; and selecting one or more of said at least two media data units on the basis of the scope information; and wherein the method is characterized by: parsing from the container file (90, 722) a first checksum, wherein the scope information indicates the media data units in the container file to which the first checksum applies; deriving a second checksum from information on said one or more of said at least two media data units; comparing the first checksum and the second checksum; and determining at least one of the following: if the first checksum is equal to the second checksum, determining that the metadata item is valid for said one or more of said at least two media data units; if the first checksum is not equal to the second checksum, determining that the metadata item is not valid for said one or more of said at least two media data units. receiving the container file (90, 722) according to a first format, the container file (90, 722) including or referring to the media data comprising at least two media data units, wherein a media data unit is a sample included in a track according to ISOBMFF or its derivatives; receiving a metadata item according to a second format different from the first format, the metadata item being linked with one or more of said at least two media data units, the container file (90, 722) including or referring to the metadata item, wherein the metadata item is either included in the container file or the metadata item is referenced in the container file, and wherein the metadata item is identified and characterized in a Meta Box inserted at a track level according to ISOBMFF or its derivatives; parsing the container file for scope information, wherein the scope information indicates said one or more of said at least two media data units; and selecting one or more of said at least two media data units on the basis of the scope information; and wherein the method is characterized by: parsing from the container file (90, 722) a first checksum, wherein the scope information indicates the media data units in the container file to which the first checksum applies; deriving a second checksum from information on said one or more of said at least two media data units; comparing the first checksum and the second checksum; and determining at least one of the following: if the first checksum is equal to the second checksum, determining that the metadata item is valid for said one or more of said at least two media data units; if the first checksum is not equal to the second checksum, determining that the metadata item is not valid for said one or more of said at least two media data units. parsing from the container file (90, 722) a first checksum, wherein the scope information indicates the media data units in the container file to which the first checksum applies; deriving a second checksum from information on said one or more of said at least two media data units; comparing the first checksum and the second checksum; and determining at least one of the following: if the first checksum is equal to the second checksum, determining that the metadata item is valid for said one or more of said at least two media data units; if the first checksum is not equal to the second checksum, determining that the metadata item is not valid for said one or more of said at least two media data units. if the first checksum is equal to the second checksum, determining that the metadata item is valid for said one or more of said at least two media data units; if the first checksum is not equal to the second checksum, determining that the metadata item is not valid for said one or more of said at least two media data units.