ACF does not identify acf vs non-acf fields. For example, you can create an image field with the field name of “_thumbnail_id” and ACF will save the value to the standard WP featured image field and then this image can be used by all the WP functions that deal with the featured image. I do this for example to require a featured image for posts and then disable the WP featured image meta box.
However, fields like title and content are different, these values are not stored in the postmeta table, they are stored in the post table and would not be effected.
You can create fields in ACF using the same meta_key as used by some other plugin and ACF will store the values in the correct place for that plugin.
The problem with this would be when two fields with the same name appear in the same forme. So, for example, if you were in the admin on the editor page for the events plugin and one of it’s fields was named “events_xyz” and you gave an acf field the same name and it appeared on the same admin page you will run into issues since both fields will be trying to save their values to the same place. Another issue may be that the other plugin might save the value in a different format than ACF stores the value. Another possible issue is that the other plugin does not actually store the values in the postmeta table, many plugins use custom db table. These are the reasons that I would go in the direction I mentioned, it chooses the safest path to prevent potential issues.
Can you use this method and get away with using the same field names for ACF as used in the other plugin? I cannot answer this question.
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