At Yale, ANSI Level 4 is used.
"Level 4 provides detailed extent of holdings information. The detailed holdings statement may be either itemized or compressed; the detailed holdings statement may be either open or closed. ... If enumeration [is] applicable, the most specific levels (including all hierarchical levels) must be included."
YUL holdings statements should always be closed. EXCEPTION: remote electronic resources use open statements when holdings are created locally. (Some vendor supplied records for electronic resources will not have serial holdings in 866)
Yale does not use ANSI Level 1 ("no indication of extent of holdings"), Level 2 ("general guidance as to the extent of the institutions' holdings") or Level 3 ("summary extent of holdings"). Note that Level 3 is understood to utilize the summary statement even when there are gaps. Cf. the examples for values 3 and 4 in MARC 21 Format for Holdings Data. Yale policy is always to indicate gaps, so to avoid ambiguity, never use first indicator 3 in 866.
Although Yale uses Level 4, the summary holdings statement is used to record holdings when there are no gaps within a range. When gaps occur, a mixed holdings statement is used, combining detailed and summary holdings statements.
The current standard is intended to allow institutions to record holdings with a combination of level 3 and level 4, allowing the institution to record some Extent of Holdings areas at the summary level, while others are recorded at the detailed level, within the same holdings statement. To simplify processing and to make clear that Yale summary statements never ignore gaps, code first indicator as 4 for all holdings statements, even when a non-gap summary statement is recorded.
For detailed scope notes on Levels 1-4, see ANSI/NISO Z39.71-1999 p. 15.