There is no rule
question: What is the rule when it comes to abbreviations that are spoken as words and letters?, context: There is no rule on what to call abbreviations whose pronunciation involves the combination of letter names and words, such as JPEG /ˈdʒeɪpɛɡ/ and MS-DOS /ˌɛmɛsˈdɒs/.  There is also some disagreement as to what to call abbreviations that some speakers pronounce as letters and others pronounce as a word. For example, the terms URL and IRA can be pronounced as individual letters: /ˌjuːˌɑːrˈɛl/ and /ˌaɪˌɑːrˈeɪ/, respectively; or as a single word: /ˈɜːrl/ and /ˈaɪərə/, respectively.[citation needed]
What is the rule when it comes to abbreviations that are spoken as words and letters?
There is no rule on what to call abbreviations whose pronunciation involves the combination of letter names and words, such as JPEG /ˈdʒeɪpɛɡ/
There is no rule on what to call abbreviations whose pronunciation involves the combination of letter names and words, such as JPEG /ˈdʒeɪpɛɡ/ and MS-DOS /ˌɛmɛsˈdɒs/.  There is also some disagreement as to what to call abbreviations that some speakers pronounce as letters and others pronounce as a word. For example, the terms URL and IRA can be pronounced as individual letters: /ˌjuːˌɑːrˈɛl/ and /ˌaɪˌɑːrˈeɪ/, respectively; or as a single word: /ˈɜːrl/ and /ˈaɪərə/, respectively.[citation needed]
<hl> There is no rule <hl> on what to call abbreviations whose pronunciation involves the combination of letter names and words, such as JPEG /ˈdʒeɪpɛɡ/
<hl> There is no rule <hl> on what to call abbreviations whose pronunciation involves the combination of letter names and words, such as JPEG /ˈdʒeɪpɛɡ/ and MS-DOS /ˌɛmɛsˈdɒs/. There is also some disagreement as to what to call abbreviations that some speakers pronounce as letters and others pronounce as a word. For example, the terms URL and IRA can be pronounced as individual letters: /ˌjuːˌɑːrˈɛl/ and /ˌaɪˌɑːrˈeɪ/, respectively; or as a single word: /ˈɜːrl/ and /ˈaɪərə/, respectively.[citation needed]
<hl> There is no rule on what to call abbreviations whose pronunciation involves the combination of letter names and words, such as JPEG /ˈdʒeɪpɛɡ/ <hl> and MS-DOS /ˌɛmɛsˈdɒs/. There is also some disagreement as to what to call abbreviations that some speakers pronounce as letters and others pronounce as a word. For example, the terms URL and IRA can be pronounced as individual letters: /ˌjuːˌɑːrˈɛl/ and /ˌaɪˌɑːrˈeɪ/, respectively; or as a single word: /ˈɜːrl/ and /ˈaɪərə/, respectively.[citation needed]