StreamField Migrations: What Do They DO!?

[rjbullock]

I don't understand what StreamField migrations do. Is the StreamField structure stored somewhere other than in the models/blocks? I understand that if you don't use a callable with a StreamField choice field (for example), that you get a "frozen" migration where the choices won't work as you might expect. Take this migration for example:

migrations.AlterField(
model_name='branchpage',
name='body',
field=wagtail.fields.StreamField([('section', wagtail.blocks.StreamBlock([('text', wagtail.blocks.RichTextBlock(features=['bold', 'italic', 'h2', 'h3', 'h4', 'ol', 'ul', 'link', 'image'], help_text='Section content.', required=False)), ('text_dated', wagtail.blocks.StructBlock([('publish_date', wagtail.blocks.DateTimeBlock(required=False)), ('depublish_date', wagtail.blocks.DateTimeBlock(required=False)),('permissions', wagtail.blocks.MultipleChoiceBlock(choices=[[17, 'Business Development Admin'], [13, 'Business Performance Admin'], [23, 'Calendar Admin'], [11, 'Community Development Admin'], [41, 'DEI Admin'], [8, 'Digital Admin'], [42, 'Digital Store Admin'], [15, 'DocuSign Admin'], [2, 'Editors'], [45, 'Employee Admin'], [39, 'Employee Discounts Admin'], [40, 'Employee Giveback'], [44, 'Employee Sync'], [27, 'Enterprise Risk Management Admin'], [3, 'Human Resources Admin'], [14, 'Information Technology Admin'], [38, 'Information Technology User'], [28, 'Innovation Admin'], [29, 'Innovation Board Manager'], [4, 'Job Descriptions'], [21, 'Job Postings'], [46, 'Job Shadow Admin'], [22, 'L360'], [16, 'Lead On User'], [18, 'Learning and Development Admin'], [20, 'Loan Servicing Admin'], [6, 'Managers'], [9, 'Marketing Admin'], [10, 'Member Experience Admin'], [12, 'Member Operations Admin'], [1, 'Moderators'], [36, 'Need to Know Admin'], [31, 'Operations Admin'], [34, 'Photo Contest Admin'], [37, 'PSO Admin'], [43, 'Q&A'], [25, 'Raffle Admin'], [30, 'Risk Mitigation Admin'], [19, 'Synapsys Campaign User'], [26, 'System Status Admin'], [33, 'Tuition Reimbursement Admin'], [24, 'UltiPro Admin'], [35, 'VIS Admin']]))]))...
)

The permissions are coming from Wagtail groups, but in the back office, if I add a new group, it doesn't become available to choose, only the ones above. So where is this data stored and referenced by Wagtail at runtime?