Problem/Motivation
When updating the options for a textfield that needs to be 'trimmed' to a certain length, it is unclear and not accurately described in the UI as to how this 'trimming' is to be performed, the only clue to the user is a numeric field (in D7/D8 the default value is 600.. and there is no indication as to what that 600 represents), it translates to 600 characters but after the textfield is actually processed the output can be very different to 600 characters because internally the process trims to the nearest sentence or paragraph, therefore it can vary considerably from what is expected depending on the text that is contained by the field.
Proposed resolution
Provide additional settings for the textfield to enable a user to describe exactly how they want the text summary to be 'trimmed' and use this configuration in the processing logic so that the resulting 'trimmed' output is representative of what the user expects.
Remaining tasks
- decide which additional fields are needed
- add fields to the formatter
- additional field processing logic
- any additional validation steps
- tests
- documentation
User interface changes
- some additional fields added to the formatter options admin UI
example screenshot:

Related Issues
#1862250: The "Trim length" format description is confusing/not clear to Drupal newcomers.
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Comments
Comment #1
catchCould be done in a minor release.
Comment #17
smustgrave commentedThank you for sharing your idea for improving Drupal.
We are working to decide if this proposal meets the Criteria for evaluating proposed changes. There hasn't been any discussion here for over 8 years which suggests that this has either been implemented or there is no community support. Your thoughts on this will allow a decision to be made.
Since we need more information to move forward with this issue, the status is now Postponed (maintainer needs more info). If we don't receive additional information to help with the issue, it may be closed after three months.
Thanks!
Comment #18
smustgrave commentedSince there's been no follow up and a feature request going to close out. Can always be re-opened
Thanks all!