In verse 12, by saying: وَنَكْتُبُ مَا قَدَّمُوا وَآثَارَ‌هُمْ (and We are recording whatever (deeds) they send ahead and whatever effects they leave behind.), the doing of deeds has been equated with 'sending ahead' to point out that good or bad deeds done in the mortal world have not gone the way of all flesh right here. Instead of that, these were their wherewithal, their baggage that has gone ahead of them to their destination in the next world that they have to confront. Now, if these deeds were good, they will make them look handsome in Paradise, and if they were bad, the embers of Hell will be their fate. The real purpose of 'recording' these deeds is to keep them preserved. Writing too is a means of doing that, so that there remains no probability of error, forgetfulness, increase or decrease.

The effects of deeds are also recorded like the deeds themselves

The terse statement: وَآثَارَ‌هُمْ (wa 'atharahum: and their effects) in Verse 12 means that the way deeds done by them are written, so are their effects too. The word: آثَارَ‌ (athar: traces, or effects) denotes fruits, outcomes and consequences that show up later and keep surviving. For example, someone helped people learn their religion, told them about its injunctions or wrote a book for to enable them to get the best out of their faith, or instituted some sort of endowment that turned out to be beneficial for people after him and, for that matter, did something that brought benefit to common Muslims, then, as far as the effects of this good deed remain, and as long as they bring fruits, they all will continue to be written in his Book of Deeds. Similar is the case of bad deeds, the evil fruits and effects of which continue to remain in the world. For example, someone instituted oppressive laws, or institutions that tend to corrupt the human deeds and morals, or made people follow the way of evil outcomes of his deed keep taking shape, these will continue to be recorded in his Book of Deeds - as it has been said by the Holy Prophet ﷺ himself while explaining this verse. Narrates Sayyidna Jarir Ibn ` Abdullah al-Bajali ؓ that the Holy Prophet ﷺ said:

مَن سَنَّ سُنَّۃً حَسَنَۃً فَلہ، اَجرُھَا وَاَجرُ مَن عَمَلَ بِھَا مِن م بَعدِہٖ مِن غَیرِ اَن یُنقَصَ مَن اُجُورِھِم شَیٔاً وَّمن سَنَّ سُنَّۃً کَانَ عَلَیہِ وزرُھا وَ وِزرُ مَن عَمِل بِھَا مِن م بَعدِہٖ لَا یُنقَصُ مِن اَوزَارِھِم شَیٔاً ، ثُمَّ تَلَا وَ نَکتُبُ مَا قَدَّمُوا وَ آثارَھُم،

Whoever started a good practice, he will have its reward, and the reward of whoever acts in accordance with it after him, without the least decrease in the reward of others acting upon it. And whoever started a bad practice, on him shall be its sin along with the sin of whoever acts in accordance with it after him, without the least decrease in the sins of others acting upon it" - Ibn Kathir from Ibn Abi Hatim.

۔

The word: آثارَ (athar: traces or effects) is also used in the sense of footprints. As in Hadith, when one walks to the Masjid, one good deed is recorded at every step he takes. Some Hadith narrations seem to indicate that the word: آثارَ (athar: traces) appearing in this verse refers to these very footprints. The way the reward of Salah is written down, similarly, on every step taken while going for Salah, one good deed is recorded. At this stage in his Tafsir, Ibn Kathir has put together all narrations that mention that there were many people in Madinah whose homes were at some distance from the Prophet's Masjid. They thought of setting up their homes closer to the Masjid. The Holy Prophet ﷺ told them not to do that. He advised them: Live where you do. When you walk in from afar, do not take this time to have been wasted. The more your steps, the more shall be your reward.

There could be a possible doubt here in that the present Surah is Makki, while the event mentioned in these Hadith belongs to Madinah. This could be answered by saying that the verse might as well be in its general sense - that is, the traces of deeds are also written - and that this verse may have been revealed in Makkah. But thereafter, when this event came to pass in Madinah, he mentioned this verse in support. Then he counted footprints too within the effects and traces the writing of which has been mentioned in this verse of the Qur'an. This way the apparent contradiction between these two explanations also stands removed (as explicated and preferred by Ibn Kathir).