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Field Access tab works very similarly to Tab Access tab, allowing to define write access up to a field granularity level.
So if a tab is editable for a role, a concrete set of fields in that can be made read-only for that role adding a new row in this tab for each field and setting false Editable Field of each of them. Or in the other way around: in a non editable tab, fields can be editable if they are added and their Editable Field property is checked.
When editing a tab with some fields defined not to be editable in this way, backend checks modifications of them from that tab to prevent this to happen. Note this also affects the field in case it was modified by for example a callout or a default expression. This is controlled by the Check on Save property, unflagging it, this check will not be performed allowing thus the field to be modified by a callout.