A tag, comprising: computer-readable memory including: an access control applet having the ability to store one or more data elements; one or more access keys; an access control list providing a mapping of the one or more access keys to the one or more data elements, wherein the access control applet implements an authentication protocol in response to receiving a write command from a writing device, the authentication protocol including confirming that the writing device provides the tag with the one or more access keys prior to allowing the writing device to write a data element to the memory; wherein the one or more access keys include a first access key and a second access key, wherein a first memory location has write permissions controlled by the first access key, and wherein a second memory location different from the first memory location has write permissions controlled by the second access key; and characterized in that a first data element is written to the first memory location by a first entity and wherein a second data element is written to the second memory location by a second entity and a pointer to at least one of the first data element and the first memory location is automatically stored together with the second data element, wherein the second entity may be the same as the first entity or different from the first entity. computer-readable memory including: an access control applet having the ability to store one or more data elements; one or more access keys; an access control list providing a mapping of the one or more access keys to the one or more data elements, wherein the access control applet implements an authentication protocol in response to receiving a write command from a writing device, the authentication protocol including confirming that the writing device provides the tag with the one or more access keys prior to allowing the writing device to write a data element to the memory; an access control applet having the ability to store one or more data elements; one or more access keys; an access control list providing a mapping of the one or more access keys to the one or more data elements, wherein the access control applet implements an authentication protocol in response to receiving a write command from a writing device, the authentication protocol including confirming that the writing device provides the tag with the one or more access keys prior to allowing the writing device to write a data element to the memory; wherein the one or more access keys include a first access key and a second access key, wherein a first memory location has write permissions controlled by the first access key, and wherein a second memory location different from the first memory location has write permissions controlled by the second access key; and characterized in that a first data element is written to the first memory location by a first entity and wherein a second data element is written to the second memory location by a second entity and a pointer to at least one of the first data element and the first memory location is automatically stored together with the second data element, wherein the second entity may be the same as the first entity or different from the first entity.