Cluster, item, in a sequence

Hi~ In the following paragraph, I don't understand the part underlined.

My questions are:

1) What does "cluster" refer to in this context? In the phone number presented as an example here, can I understand "919" as a cluster and that the phone number consists of three clusters, that is "919", "555", and "3290"?

2) In this context, what does "item" refer to ?

3) 2) What does the phrase "in a sequence" mean?

Thanks a lot~!

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The limitations of working memory can be somewhat circumvented by the ability to “chunk” information. Chunking is the presenting of information in small bits to assist the working memory’s capacity for recall. A
chunk is a grouping of information in which no cluster exceeds three to five items in a sequence. The most recognizable example of chunking is a standard phone number. Rather than 9195553290, it is presented as
919-555-3290.