A method of processing multi-layer video data using an ISO base media file format, ISOBMFF, or a file format based on an ISOBMFF, the method comprising: generating, in a file storing a multi-layer bitstream comprising a sequence of bits that forms a representation of pictures of the video data, a track box that contains metadata for a track, the track containing media content, the media content of the track comprising a sequence of samples, wherein generating the track box comprises: generating, in the track box, a sample description box containing a sample group description entry; and generating, in the track box, a sample-to-group box for the track, the sample-to-group box mapping samples of the track into a sample group, the sample group comprising samples sharing a property specified by the sample group description entry, the sample-to-group box specifying target layers among layers present in the track based on a version of the sample group description box and the sample-to-group box being equal to 1, the sample-to-group box specifying the target layers by using a grouping type parameter that specifies a target layers value that specifies the indicated target layers among all the layers present in the track containing the sample group and all layers referred to, directly and indirectly, by the layers that are present in the track containing the sample group, each of the target layers containing at least one picture belonging to a particular picture type, and the sample group is one of: a temporal sub-layer access sample group and the particular picture type is a temporal sub-layer access picture type, or a stepwise temporal sub-layer access (STSA) sample group and the particular picture type is a step-wise temporal sub-layer access picture type. generating, in a file storing a multi-layer bitstream comprising a sequence of bits that forms a representation of pictures of the video data, a track box that contains metadata for a track, the track containing media content, the media content of the track comprising a sequence of samples, wherein generating the track box comprises: generating, in the track box, a sample description box containing a sample group description entry; and generating, in the track box, a sample-to-group box for the track, the sample-to-group box mapping samples of the track into a sample group, the sample group comprising samples sharing a property specified by the sample group description entry, the sample-to-group box specifying target layers among layers present in the track based on a version of the sample group description box and the sample-to-group box being equal to 1, the sample-to-group box specifying the target layers by using a grouping type parameter that specifies a target layers value that specifies the indicated target layers among all the layers present in the track containing the sample group and all layers referred to, directly and indirectly, by the layers that are present in the track containing the sample group, each of the target layers containing at least one picture belonging to a particular picture type, and the sample group is one of: a temporal sub-layer access sample group and the particular picture type is a temporal sub-layer access picture type, or a stepwise temporal sub-layer access (STSA) sample group and the particular picture type is a step-wise temporal sub-layer access picture type. the sample-to-group box mapping samples of the track into a sample group, the sample group comprising samples sharing a property specified by the sample group description entry, the sample-to-group box specifying target layers among layers present in the track based on a version of the sample group description box and the sample-to-group box being equal to 1, the sample-to-group box specifying the target layers by using a grouping type parameter that specifies a target layers value that specifies the indicated target layers among all the layers present in the track containing the sample group and all layers referred to, directly and indirectly, by the layers that are present in the track containing the sample group, each of the target layers containing at least one picture belonging to a particular picture type, and the sample group is one of: a temporal sub-layer access sample group and the particular picture type is a temporal sub-layer access picture type, or a stepwise temporal sub-layer access (STSA) sample group and the particular picture type is a step-wise temporal sub-layer access picture type. a temporal sub-layer access sample group and the particular picture type is a temporal sub-layer access picture type, or a stepwise temporal sub-layer access (STSA) sample group and the particular picture type is a step-wise temporal sub-layer access picture type.