What is the difference between ENCODE Tier 1, 2 and 3 cell types? The ENCODE Experiment Matrix at UCSC lists the different available cell types under the categories "Tier 1", "Tier 2" and "Tier 3". What is the difference between these classifications?

What, for example, makes GM12878 a Tier 1 cell type and A549 a Tier 2 cell type?
The tiers denote the original priority of sequencing and processing the samples. This is understandable given the number of marks and experiment types that ENCODE tried to sequence. Further details are available on ENCODE's website. 

Edit: Just to expand a bit, tier one cells were supposed to get processed first. Then tier 2 came second. Of course tier 2 had a LOT more cells, so it got split into 2 and 2.5, since there were already apparently tier 3 cells.